P See P value.

PAHO See Pan American Health Organization.

PAIRED SAMPLES In a clinical trial, pairs of subject patients may be studied. One member of each pair receives the experimental regimen and the other receives a suit- ably designated control regimen. Pairing should be based on a prognostic variable, such as age.

Pairing may similarly be used in a case-control study or in a cohort study. See also matching.

PAN AMERICAN HEALTH ORGANIZATION (PAHO) The autonomous division of the World Health Organization (WHO) that deals with the health affairs of North, Cen- tral, and South America (www.paho.org).

PANDEMIC An epidemic occurring worldwide or over a very wide area, crossing international boundaries, and usually affecting a large number of people.

PANEL STUDY A combination of cross-sectional and cohort methods in which the inves- tigator conducts a series of cross-sectional studies of the same individuals or study sam- ple. This method of study permits changes in one variable to be related to changes in other variables.

PARADIGM A broad intellectual framework or set of assumptions used to analyze a scientific issue or a field of scientific inquiry. Loosely, a pattern of thought or conceptualization—an overall way of regarding phenomena within which scientists normally work. In the particular sense used by Thomas Kuhn, paradigms are governing concepts of cause in a given science during a given period. Paradigms reflect causal concepts operative in current or so-called “normal” science. In Kuhn’s influential view (one not accepted by all philosophers of science), such paradigms can be displaced only by scientific revolutions.302 The causal theories of disease governing the thought of successive eras reflect different paradigms (e.g., miasma, germs, multiple causation). The inductive methods of Aristotle in the fourth century b.c. and of Bacon in the seventeenth century were rejected by David Hume in the eighteenth century and again by Karl Popper in the twentieth. However, they have been given modern forms in the machine-learning, artificial intelligence, and Bayesian literature. A paradigm may dictate what form of explanation will be found acceptable or well supported, but a science may change paradigms.7,196 See also deductive logic; hypothetico-deductive method; inductive logic.

PARAMETER In mathematics, a quantity in a formula or model that is assumed not to vary within the system under study. In statistics and epidemiology, a measureable characteristic of a population that is often estimated by a statistic, e.g., mean, standard deviation, regression coefficients.

PARAMETRIC TEST A statistical test that depends upon assumption(s) about the distribution of the data (e.g., that these are normally distributed).

PARAMUTATION In epigenetics, an interaction between two alleles of a single locus, resulting in a heritable change of one allele that is induced by the other allele. Paramutation violates Mendel’s first law, which states that in the process of the formation of the gametes (egg or sperm) the allelic pairs separate, one going to each gamete, and that each gene remains completely uninfluenced by the other. In paramutation, an allele in one generation heritably affects the other allele in future generations, even if the allele causing the change is itself not transmitted. What may be transmitted in such a case are different types of RNAs, which may be packaged in egg or sperm and cause paramutation upon transmission to the next generation. This may mean that RNA is a molecule of inheritance, just like DNA. Paramutation can result in a single allele of a gene con- trolling a spectrum of phenotypes. See also epigenetic inheritance.

PARASITE An animal or vegetable organism that lives on or in another and derives its nourishment therefrom. An obligate parasite is one that cannot lead an independent nonparasitic existence. A facultative parasite is one that is capable of either parasitic or independent existence.

PARASITE COUNT See worm count.

PARASITE DENSITY The collective degree of parasite load (or of parasitemia) in a population. Calculated by the geometric mean or the weighted average of the individual parasite counts (e.g., by using a frequency distribution based on a geometric progression).

PARASITEMIA Presence of parasites in the blood. The term can also be used to express the quantity of parasites in the blood (e.g., “a parasitemia of 2%”).

PARATENIC HOST (Syn: transport host) A second, third, or subsequent intermediate host of a parasite, in which the parasite does not undergo any development or replica- tion but remains, usually encysted, until the paratenic host is ingested by the definitive host of the parasite.

PARITY The status of a woman with regard to having borne viable children. The number of full-term children previously borne by a woman, excluding miscarriages and abor- tions in early pregnancy but including stillbirths.

PARTICIPANT Person upon whom research is conducted. The term research participant is suggested in preference to research subject on the grounds that subject may be demean- ing, but this can be ambiguous, because members of research teams are also called par- ticipants. Volunteer may be an alternative, but this too can be misleading, because not all persons upon whom research is conducted are volunteers. The term research subject is less ambiguous. The most suitable term differs according to the setting and should be selected for both clarity and for acceptability in that setting.

PARTICIPANT OBSERVATION A method used in the social sciences in which the research worker (observer) is (or pretends to be) a member of the group being studied. Epidemiologists distrust the method on the grounds that objectivity of the observations may be compromised.

PARTICULARIZATION A method of analysis opposite to generalization or abstraction. It focuses on the specificity of a number of facts and illustrates an issue through the use of example.

PASSAGE The transfer of microorganisms from human to animal host(s) either directly or via laboratory culture; in the laboratory, this procedure is used to check for conform- ity with the Henle–Koch postulates.

PASSENGER VARIABLE A variable that varies systematically with the dependent (outcome) variable under study without having a direct causal relation to it.22 A third (explanatory) variable, the common cause of both the dependent and the passenger variable, “explains,” or accounts for, their association.

PASSIVE SMOKING See involuntary smoking; environmental tobacco smoke.

PASTEURIZATION The process of heat-treating milk or other perishable foodstuffs to kill pathogens. Developed by and named for the great French bacteriologist Louis Pasteur (1822–1895).

PATH ANALYSIS A mode of analysis involving assumptions about the direction of causal relationships between linked sequences and configurations of variables. This permits the analyst to construct and test the appropriateness of alternative models (in the form of a path diagram) of the causal relations that may exist within the array of variables included in the finite system studied. Identification of the less probable sequences of causal pathways may permit them to be eliminated from further consideration.

PATH DIAGRAM See causal diagram.

PATHOGEN An organism capable of causing disease (literally, causing a pathological process).

PATHOGENESIS The mechanisms by which a cause or etiological agent produces disease. The etiology of a disease, disability or other health state begins with causes that initiate pathogenesis or favor pathogenetic mechanisms; control of such causes favours primary prevention of the disease.

PATHOGENICITY The property of an organism that determines the extent to which overt disease is produced in an infected population, or the power of an organism to produce disease. Also used to describe comparable properties of toxic chemicals, etc. Pathogenicity of infectious agents is measured by the ratio of the number of persons developing clinical illness to the number exposed to infection. See also virulence, with which pathogenicity is sometimes confused.

PEARSON’S CHI-SQUARE See chi-square test.


PEDIGREE A diagram showing the ancestral relationships and transmission of genetic traits over several generations of a family.

PEER REVIEW Process of review of research proposals, manuscripts submitted for publication, and abstracts submitted for presentation at scientific meetings, whereby these are judged for scientific and technical merit by other scientists in the same field. The term also refers to review of clinical performance when it is a form of medical audit.

PENALIZED ESTIMATION See shrinkage estimation.

PENETRANCE See genetic penetrance.

PERCEIVED NEED A felt need. The term usually refers to the need for health care felt by the person or community concerned and which may not be perceived by health professionals or political authorities.

PERCENTILE The set of divisions that produce exactly 100 equal parts in a series of continuous values, such as children’s heights or weights. Thus, a child above the 90th percentile has a greater value for height or weight than over 90% of all in the series.


  1. Systematic differences in the care provided to members of the different studygroups other than the intervention under investigation. For instance, patients who know they are in the control group may be more likely to use other forms of care, patients who know they are in the intervention group may be more likely to experience placebo effects, and health care providers may treat patients differently according to what group they are in. To prevent unintended differences in care and placebo effects, those providing and receiving care can sometimes be “blinded” so that they do not know the group to which the recipients of care have been allocated. See also blind(ed) study.
  2. It may exist when the intervention actually received by study subjects differed substantially from the intervention that was intended or planned.

PERINATAL MORTALITY Literally, mortality around the time of birth. Conventionally, this time is limited to the period between 28 weeks gestation and 1 week postnatally. However, as the following discussion indicates, other factors, especially the weight of the fetus, should be considered. The Ninth (1975) Revision of the International Classifi- cation of Diseases includes the following:

Perinatal mortality statistics

It is recommended that national perinatal statistics should include all fetuses and infants delivered weighing at least 500 g [or, when birth weight is unavailable, the corresponding gestational age (22 weeks) or body length (25 cm crown–heel)], whether alive or dead. It is recognized that legal requirements in many countries may set different criteria for registration purposes, but it is hoped that countries will arrange the registration or reporting procedures in such a way that the events required for inclusion in the statistics can be identified easily. It is further recommended that less mature fetuses and infants should be excluded from perinatal statistics unless there are legal or other valid reasons to the contrary.

It is recommended above that national statistics would include fetuses and infants weighing between 500 g and 1000 g both for their inherent value and because their inclusion improves the completeness of reporting at 1000 g and over.

Inclusion of this group of very immature births, however, disrupts international comparisons because of differences in national practices concerning their registration. Another factor affecting international comparisons is that all live-born infants, irrespective of birth weight, are included in the calculation of rates, whereas some lower limit of maturity is applied to infants born dead.

In order to eliminate these factors, it is recommended that countries should present, solely for international comparisons, “standard perinatal statistics” in which both the numerator and denominator of all rates are restricted to fetuses and infants weighing 1000 g or more [or, where birth weight is unavailable, the corresponding gestational age (28 weeks) or body length (25 cm crown–heel)].

The Conference for the tenth revision (ICD-10) made no changes to these definitions.


1. In most industrially developed nations, this is defined as

2. The WHO’s definition, more appropriate in nations with less well established vital records, is

Note the difference in denominator of the perinatal mortality rate as defined by the WHO and in industrially developed nations. This makes international comparison difficult. The WHO Expert Committee on the Prevention of Perinatal Mortality and Morbidity (1970) recommended a more precise formulation: “Late fetal and early neonatal deaths weighing over 1000 g at birth expressed as a ratio per 1000 live births weighing over 1000 g at birth.”

PERIODICITY A repeating pattern of a phenomenon or an event, especially the repeti- tion of comparable values, e.g., seasonal fluctuation in numbers of cases of respiratory infections.

PERIODIC (MEDICAL) EXAMINATIONS Assessment of health status conducted at pre- determined intervals (e.g., annually or at specified milestones in life, such as infancy, school entry, preemployment, or preretirement). This form of medical examination generally follows a formal protocol, employing a set of structured questions and/or a predetermined set of laboratory tests.

PERIOD OF COMMUNICABILITY See communicable period.

PERMISSIBLE EXPOSURE LIMIT (PEL) An occupational health standard to safeguard workers against dangerous substances in the workplace. See safety standards.

PERSONAL HEALTH CARE Those services to individuals that are performed on a one-to-one basis by a health care worker for the purpose of maintaining or restoring health.

PERSONAL MONITORING DEVICE An instrument attached to a person to measure the exposure of that person to hazardous substance(s).

PERSON-TIME A measurement combining persons and time as the denominator in incidence and mortality rates when, for varying periods, individual subjects are at risk of developing disease or dying. It is the sum of the periods of time at risk for each of the subjects. The most widely used measure is person-years. With this approach, each subject contributes only as many years of observation to the population at risk as the period over which that subject has been observed to be at risk of the disease; a subject observed over 1 year contributes 1 person-year, a subject observed over a 10-year period contributes 10 person-years. This method can be used to measure incidence rate over extended and variable time periods. See also context.

PERSON-TIME INCIDENCE RATE (Syn: interval incidence density) A measure of the incidence rate of an event (e.g., a disease or death) in a population at risk, given by Number of events occurring during the interval Number of person-time units at risk observed during the interval

PERSON-TO-PERSON SPREAD OF DISEASE See transmission of infection.

PERSON-YEARS See person-time.

PERSPECTIVE PLOT Diagrammatic representation of the relationship among three variables, one each on the horizontal and vertical axes and the third represented by a series of lines drawn so as to convey an illusion of three dimensions. See also contour plot.

PHARMACOEPIDEMIOLOGY (Syn: drug epidemiology) The study of the distribution and determinants of drug-related events in populations and the application of this study to efficacious treatment. The application of epidemiological knowledge, methods, and reasoning to describe, explain, control, and predict the uses and effects (beneficial and adverse) of drugs, vaccines, and related biological products in human populations. The public health foundation of pharmacoepidemiology is that drugs and vaccines are among the factors that influence the distribution of health states in human populations. Its core lies at the intersection of two subspecialties: clinical pharmacology and clinical epidemiology. Pharmacoepidemiology also aids pharmacology, public health, and other health sciences by increasing knowledge about the occurrence and causes of diseases, the distribution of health states, and the functioning of the health care system92,303,304 See also cancer epidemiology; cardiovascular epidemiology; environmental epide- miology; molecular epidemiology; neuroepidemiology.

PHASE I, PHASE II, PHASE III TRIAL See clinical trial.

PHENOTYPE The observable properties, characteristics, or form of an organism or person produced by the genotype in synergy or interaction with the environment. All aspects of a living organism other than its genetic constitution. See also genetic penetrance.

PHYSICIAN (Syn: medical practitioner, doctor) Professional person qualified by education and authorized by law to practice medicine.

PICKLES CHARTS Day-by-day plots of new cases of infectious disease seen in the practice of a family doctor; the method was developed and used by the British general

practitioner and epidemiologist William Pickles (1885–1969) to demonstrate clus- tering in time and the progress of epidemic diseases in a small, relatively isolated, closed community. It has been used frequently by doctors and nurses in other isolated communities.305

PIE CHART A circular diagram divided into segments, each representing a category or subset of data. The amount for each category is proportional to the angle subtended at the center of the circle and hence to the area of the sector.

When several pie charts are used to describe several populations, the area of each circle is proportional to the size of the population it represents.

PILOT INVESTIGATION, STUDY A small-scale test of the methods and procedures to be used on a larger scale if the pilot study demonstrates that these methods and procedures can work.

PLAs Persons living with AIDS.

PLACEBO A medication or procedure that is inert (i.e., one having no pharmacological effect) but intended to give patients the perception that they are receiving treatment or assistance for their complaint. From Latin placebo, “I shall please.”

PLACEBO EFFECT The beneficial effect resulting solely from the administration of a treatment, no matter whether strictly a placebo, an active drug, or another therapeutic procedure. Hence, placebo effects accompany also the prescription of efficacious drugs. They may be due to a variety of factors and occur through different mechanisms, including an empathic relationship between the prescribing physician and the patient or the patient’s expectation of an effect (e.g., the power of suggestion). The very accomplishments of medicine, attained through scientific research, have augmented its placebo effects and metaphoric power.127 See also halo effect; nocebo.

PLASTICITY The potential for change in intrinsic characteristics in response to envi- ronmental stimuli.16 Phenotypic plasticity is a cell’s ability to change its behavior in response to internal or external environmental cues.116 The ability of a genotype to produce more than one alternative form of structure, physiological state, or behavior in response to environmental conditions. It is a quality of organisms as they develop.

Within the limits imposed by genetic and mechanical constraints, each person has a range of options for his or her life course and final body form and function.45 See also developmental origins hypothesis; epigenetics; programming; sensitive period.

PLAUSIBILITY See biological plausibility; coherence; Hill’s criteria of causation.

PLOT, BOX AND WHISKER See box-and-whiskers plot.

PLOT, CAT AND WHISKER See cat-and-whiskers plot.

PLOT, FOREST See forest plot.

PLOT, FUNNEL See funnel plot.

POINT SOURCE EPIDEMIC See epidemic, common source.

POISSON DISTRIBUTION A distribution function used to describe the occurrence of rare events or the sampling distribution of isolated counts in a continuum of time or space (e.g., cases of an uncommon disease). The number of events X has a Poisson distribution with parameter λ (lambda) if the probability of observing k events (k = 0,1,…) is equal to

where e is the base of natural logarithm, 2.7183 ….The mean and variance of the distribution are both equal to λ. This distribution is used in modeling person-time incidence rates and caseloads.

POLICY A guide to action to change what would otherwise occur; a decision about amounts and allocations of resources; a statement of commitment to certain areas of concern; the distribution of the amount shows the priorities of decision makers. Public policy is policy at any level of government.104,229 See also healthy public policies.

POLITICAL EPIDEMIOLOGY The scientific study of political factors, processes, and conditions affecting the health of human populations.306 The effects on health of the institutions derived from political power.307 At least since Rudolf Virchow (1821–1902) proposed medicine as a political science, many epidemiologists have studied the effects on individual and community health of politically modifiable processes as democracy, political rights, and civil liberties. Political economy is the discipline that examines the relationships of individuals to society, economic production, and the activities of the state. Political economy of health is a theoretical framework used to study health in- equalities; it proposes that health disparities and inequalities are caused by social structures and institutions, which create, enforce, and perpetuate poverty and privilege.137.308 Political ecology is the study of the political and economic principles controlling the relations of human beings to one another and to the environment.18

POLLUTANT Any undesirable solid, liquid, or gaseous matter in a solid, liquid, or gaseous environmental medium.

POLLUTION Any undesirable modification of air, water, or food by substances that are toxic or may have adverse effects on health or that are offensive even if not necessarily harmful to health. See also contamination.

POLYGENIC DISEASES Diseases in which multiple gene variants or genetic alterations jointly influence the risk of developing the disease, often through complex processes in which environmental and epigenetic factors also intervene. A term sometimes used to refer to diseases of complex etiology. An antonym of monogenic diseases.

POLYGENIC INHERITANCE The transmission of a phenotypic trait whose expression depends on the effect of multiple genes.

POLYMORPHISM, GENETIC The occurrence in a population of two or more genotypes. The existence of two or more genetic variants. A genome segment or locus in which alternate forms (alleles) are present. In population genetics, variation is polymorphic if all alleles are found at frequencies >1%. Genetic polymorphims can have opposite relations with different diseases; e.g., people with the NAT2-slow genotype are thought to have an increased risk of bladder cancer if exposed to aromatic amines (which are deactivated by acetylation) but a decreased risk of colon cancer if exposed to heterocy- clic amines (activated by acetylation).23,214–219,309

POLYTOMOUS Divided or involving division into multiple parts. A categorical variable with three or more categories. See also measurement scales.

PONDERAL INDEX The anthropometric index of body mass. Defined as height divided by the cube root of the body weight. The body mass index is generally regarded as a better index of body mass because it appears better correlated with tissue composition (percent body fat).


  1. All the inhabitants of a given country or area considered together; the number of inhabitants of a given country or area.
  2. In sampling, the whole collection of units (the “universe”) from which a sample may be drawn; not necessarily a population of persons – the units may be institutions, records, or events. The sample is intended to give results that are representative of the whole population; it may deviate from that goal owing to random and systematic errors. See also general population.

POPULATION ATTRIBUTABLE RISK (PAR) This term is sometimes used62,310,311 as a synonym for attributable fraction (population). It is also used for the difference of the population rate or risk of disease and the rate or risk in the unexposed.

POPULATION ATTRIBUTABLE RISK PERCENT This is the attributable fraction in the population expressed as a percentage. See also attributable fraction (population).

POPULATION BASED Pertaining to a general population defined by geopolitical boundaries; this population is the denominator and/or the sampling frame.

POPULATION DYNAMICS Changes in the structure of a population; loosely used as a synonym for demography.

POPULATION EXCESS RATE A measure of the number of disease cases associated with exposure to a putative cause of the disease in the population. It is the difference between the rates of disease in the entire population and among the nonexposed.

POPULATION GENETICS Study of the genetic composition of populations. The main aim is to estimate gene frequencies and detect selective factors in the environment that influence these frequencies.


  1. The health of the population measured by health status indicators; it is influenced by physical, biological, social, and economic factors in the environment, by personal health behavior, and by access to and effectiveness of health care services.
  2. The prevailing or aspired level of health in the population of a specified country or region or in a defined subset of that population. The distinction between population health and public health is that population health describes the condition whereas public health includes the policies, programs, practices, procedures, institutions,
    and disciplines required to achieve the desired state of population health. The term also sometimes means the disciplines involved in studying the determinants and dynamics of a population’s health status.


POPULATION MEDICINE See community medicine.

POPULATION MOMENTUM In a growing population, the phenomenon of continuing population growth beyond the time when replacement-level fertility has been achieved because of the increasing size of childbearing and younger age cohorts resulting from higher fertility and/or falling mortality in preceding years.

POPULATION PREVENTIVE STRATEGY See strategy, “population.”

POPULATION PYRAMID A graphic presentation of the age and sex composition of the population. The population pyramid is constructed by computing the percentage distribution of a population simultaneously cross-classified by sex and age. The percentage that each female age group is of the total is plotted on the right and the corresponding percentages for males are plotted on the left. Sometimes the pyramid is constructed using absolute numbers, rather than proportions, in each age and sex group. A population pyramid is intended to provide a quick overall comprehension of age and sex structure in the population. A population whose pyramid has a broad base and narrow apex may be identified as a high-fertility population. Changing shape over time reflects the changing composition of the population associated with changes in fertility and mortality at each age. Since the figure is two-dimensional, the word pyramid is incorrectly used, but the more accurate word profile has never caught on.

POPULATION, SOURCE (Syn: base population) The group from which a study group is selected.

POPULATION, STUDY (Syn: study group, study sample) The group selected for investigation.

POPULATION, TARGET See target population.

POPULATION THINKING By contrast to individual thinking, a mode of reasoning that consists in first observing and then predicting the experiences of a whole group of people defined in a specific way (e.g., geographically, socially, biologically).10 Population thinking is deemed indispensable for group comparisons. See also clinical study; prevention paradox; strategy, “population.”

POSITION EFFECT The effect on the expression of a gene when its location in a chromo- some is changed, often by translocation. See also epigenetic inheritance.

POSITIVE PREDICTIVE VALUE See predictive value, positive.

POSTERIOR ODDS The odds calculated after reference to results of a study. See Bayes’ theorem.

POSTERIOR PROBABILITY The probability calculated after reference to results of a study. See Bayes’ theorem.

POSTMARKETING SURVEILLANCE Studies conducted after a drug or vaccine has been licensed for marketing and public use. Designed to provide information on the actual uses and effects of the product under common conditions of living, especially on the occurrence of side effects and adverse drug reactions unlikely to be detected with the lower numbers of subjects that take part in premarketing studies. It uses epidemio- logical and nonepidemiological designs. The latter include voluntary reporting systems of adverse events by health professionals. Postmarketing epidemiological studies may be observational and experimental.92,303 See also clinical trial, phase iv.

POSTNEONATAL MORTALITY RATE The number of infant deaths between 28 days and 1 year of age in a given year per 1000 live births in that year. It is an important rate to monitor in developing countries, where older infants frequently die of infections and malnutrition.

POTENCY The strength of a particular drug, toxin, or hazard; the ratio of the dose of a standard amount required to elicit a specific response to the dose of the test agent that elicits the same response.

POTENTIAL OUTCOMES, POTENTIAL OUTCOME MODEL A potential outcome is the outcome of a person, group, or other unit of study under a possibly hypothetical sequence of events.91 For example, it may be hypothesized that a person will survive at least 5 more years by accepting a treatment but not survive by declining the treatment. Under this model for the person’s potential outcomes, survival is a hypothesized out- come under treatment. If the person declines treatment and then dies, we will not know if indeed the model is correct because we will not observe what his or her outcome would have been had the treatment been accepted. Any further reasoning about the person’s outcome under treatment will thus involve counterfactual logic.

POTENTIAL YEARS OF LIFE LOST (PYLL) A measure of the relative impact of vari- ous diseases and lethal forces on society. PYLL highlights the loss to society as a result of youthful or early deaths. The figure for PYLL due to a particular cause is the sum, over all persons dying from that cause, of the years that these persons would have lived had they reached a specified age. The concept derives from Petty’s Political Arithmetic (1687) and is elaborated upon in Dublin and Lotka’s Money Value of a Man (1930).

POVERTY The individual’s or group’s lack of material or cultural resources. Inability to afford an adequate standard of living. See also absolute poverty level; relative poverty level.

POWER Roughly, the ability of a study to demonstrate an association if one exists. The power of a study is determined by several factors, including the frequency of the condi- tion under study, the magnitude of the effect, the study design, and sample size. Math- ematically, power is defined as the probability that the null hypothesis will be rejected if it is false, and is equal to 1 – β, where β is the probability of type ii error (failing to reject a false null hypothesis).

PRAGMATIC STUDY, PRAGMATIC TRIAL (Syn: management trial) A study (including randomized clinical trials) whose aim is to improve health status or health care of a specified population, provide a basis for decisions about health care, or evaluate previ- ous actions.14,255,256 See also explanatory study; community diagnosis; program review.

PRECAUTIONARY PRINCIPLE The “better safe than sorry” approach to assessing and managing health risks, especially those associated with environmental hazards. Where there is sufficient evidence to believe that a risk exists, prudence and ethical norms and values require that action should be taken to reduce or eliminate that risk, even if the evidence is not conclusive. Similar in some ways the medical maxim Primum non nocere (“First do no harm”) and the ethical principle of nonmaleficence.

PRECIPITATING FACTORS See causation of disease, factors in.


  1. 1      Relative lack of random error. Contrast with internal validity, the relative lack of bias, or nonrandom error. As a principle, in etiological research, internal validity must take precedence over precision. However, sometimes a slightly biased, highly precise estimate may be preferable to an unbiased but highly imprecise estimate.
  1. In statistics, one measure of precision is the inverse of the variance of a measurement or estimate. A measure of imprecision is the standard error of measurement — the standard deviation of a series of replicate determinations of the same quantity.
  2. The quality of being sharply defined or stated. One measure of precision is the number of distinguishable alternatives from which a measurement was selected, sometimes indicated by the number of significant digits in the measurement. Precision does not imply accuracy. See also measurement, terminology of.

PRECURSOR A condition or state preceding pathological onset of a disease; sometimes detectable by screening. See also risk marker.

PREDICTIVE VALUE (of a screening test or of a diagnostic test) The probability of the disease given the results of the test. Predictive values of a test are determined by the sensitivity and specificity of the test and by the prevalence of the condition for which the test is used. See also screening.

PREDICTIVE VALUE, NEGATIVE (Syn: predictive value of a negative test) The prob- ability that a person with a negative test result is a true negative (e.g., does not have the disease).

PREDICTIVE VALUE, POSITIVE (Syn: predictive value of a positive test) The probability that a person with a positive test result is a true positive (e.g., does have the disease).

PREDISPOSING FACTORS See causation of disease, factors in.

PREGNANCY-RELATED DEATH See maternal mortality.

PREMUNITION A state of resistance in a host harboring a parasite to superinfection by a parasite of the same species. The state is dependent on the continued survival of para- sites in the body and disappears after their elimination; it may be complete or partial. A term used mainly in the epidemiology of parasitic diseases, especially malaria.

PREPATENT PERIOD In parasitology, the period equivalent to the incubation period of microbial infections; the corresponding phase may be biologically different from microbial multiplication when the invading organism is a multicellular parasite that undergoes developmental stages in the host.


PRESUMPTIVE TREATMENT Treatment of clinically suspected cases without, or prior to, results from confirmatory laboratory tests.

PREVALENCE A measure of disease occurence: the total number of individuals who have an attribute or disease at a particular time (it may be a particular period) divided by the population at risk of having the attribute or disease at that time or midway through the period. When used without qualification, the term usually refers to the situation at a specified point in time (point prevalence).A measure of occurrence or disease frequency, often used to refer to the proportion of individuals in a population who have a disease or condition.12,31,97,313 It is a proportion, not a rate. Types of prevalence include the following:

Annual prevalence The proportion of individuals with the disease or attribute at any time during a year. It includes cases of the disease arising before but extending into or through the year as well as those having their inception during the year. Only occasionally used.

Lifetime prevalence The proportion of individuals who have had the disease or condition for at least part of their lives at any time during their lifecourse. One-year prevalence The proportion of individuals with the disease or condition at any time during a calendar year. It includes cases arising before and during the year.

Period prevalence The proportion of individuals with a disease or an attribute at a specified period of time. To calculate a period prevalence, the most appropriate denominator for the period must be found.

Point prevalence The proportion of individuals with a disease or an attribute at a specified point in time.

PREVALENCE POOL The diseased subset of a population.12

PREVALENCE STUDY See cross-sectional study.

PREVENTION Actions that prevent disease occurrence. Actions aimed at eradicating, eliminating, or minimizing the impact of disease and disability, or if none of these is feasible, retarding the progress of disease and disability. The concept of prevention is best defined in the context of levels of prevention, traditionally called primary, secondary, and tertiary prevention. Other levels (primordial prevention, quaternary prevention) are also used. There is significant conceptual and practical overlapping among levels – largely, depending on the type of disease (e.g., on the natural history of the disease).

Effective prevention strategies often interact and operate across levels.

  1. Primordial prevention consists of conditions, actions, and measures that minimize hazards to health and that hence inhibit the emergence and establishment of processes and factors (environmental, economic, social, behavioral, cultural) known to increase the risk of disease.313 Primordial prevention is accomplished through many public and private healthy public policies and intersectoral action. It may be seen as a form of primary prevention.
  2. Primary prevention aims to reduce the incidence of disease by personal and communal efforts, such as decreasing environmental risks, enhancing nutritional status, immunizing against communicable diseases, or improving water supplies. It is a core task of public health, including health promotion.
  3. Secondary prevention aims to reduce the prevalence of disease by shortening its duration. If the disease has no cure, it may increase survival and quality of life; it will also increase the prevalence of the disease. It seldom prevents disease occurrence; it does so only when early detection of a precursor lesion leads to complete removal of all such lesions. It is a set of measures available to individuals and communities for the early detection and prompt intervention to control disease and minimize disability; e.g., by the use of screening programs. It is a core task of preventive medicine. Both early clinical detection and population-based screening usually aim at achieving secondary prevention. In certain diseases, these activities may also contribute to tertiary prevention.
  4. Tertiary prevention consists of measures aimed at softening the impact of long- term disease and disability by eliminating or reducing impairment, disability, and handicap; minimizing suffering; and maximizing potential years or useful life. It is mainly a task of rehabilitation.
  5. Quaternary prevention consists of actions that identify patients at risk of overdiagnosis or overmedication and that protect them from excessive medical intervention.314 Actions that prevent iatrogenesis.

PREVENTIONIST A physician who specializes in preventive medicine and possesses specialist qualifications in this field. Loosely, a professional who practices preventive medicine.315


PREVENTION LEVEL See prevention.

PREVENTION PARADOX As formulated by Geoffrey Rose (1926–1993), a preventive measure that brings large benefits to the community but may offer little to each participating person.72 For example, to prevent one death due to a motor vehicle accident, many hundreds of people must wear seat belts. Conversely, an intervention that brings much benefit to an individual may have a small impact in the population. See also strategy.

PREVENTIVE MEDICINE The application of preventive measures by clinical practitioners. A specialized field of medical practice composed of distinct disciplines that utilize skills focusing on the health of defined populations in order to promote and maintain health and well-being and prevent disease, disability, and premature death.

In addition to the knowledge of basic and clinical sciences and the skills common to all physicians, the distinctive aspects of preventive medicine include knowledge of and competence in biostatistics; epidemiology; administration, including planning, organization, management, financing, and evaluation of health programs; environmental health; application of social and behavioral factors in health and disease; and the application of primary, secondary, and tertiary prevention measures within clinical medicine. See also health education; health promotion; strategy.

PRIMARY CASE The individual who introduces the disease into the family or group under study. Not necessarily the first diagnosed case in a family or group. See also index case.

PRIMARY CARE EPIDEMIOLOGY The application of epidemiological principles and methods to the study of problems arising in primary care, aimed at improving their management. This subdiscipline includes studies at and on the interface between pri- mary care and the community (or the general population) as well as on the interfaces between primary and secondary (and tertiary) care. Much work involves studying the determinants and outcomes of consultations in primary care. Here, determinants include also the nature of symptoms, signs, or illnesses occurring in the community and factors influencing decisions to consult or not to consult. Outcomes include — in addition to all other conventional outcomes—the duration, severity, and impact of illnesses and symptom complexes. They also include all aspects of primary care management (e.g., access, diagnostic investigations, referrals, treatments).316 As mentioned above, epidemiology is applied in many types of populations and health care settings; hence this dictionary includes just some examples of definitions for a few setting-based branches of epidemiology.


  1. Health care that begins at the time of first encounter between a patient and a provider of health care. It often is primary medical care.317
  2. Primary health care is essential health care made accessible at a cost the country and the community can afford, with methods that are practical, scientifically sound, and socially acceptable.318 Everyone in the community should have access to it. Related sectors should also be involved in it in addition to the health sector. At the very least it should include education of the community on the health problems prevalent and on methods of preventing health problems from arising or of controlling them; the promotion of adequate supplies of food and of proper nutrition; safe water and basic sanitation; maternal and child health care, including family planning; the prevention and control of locally endemic diseases; immunization against the main infectious diseases; appropriate treatment of common diseases and injuries; and the provision of essential drugs. See also community-oriented primary health care.


PRINCIPAL COMPONENT ANALYSIS A statistical method to simplify the description of a set of interrelated variables. Its general objectives are data reduction and interpre- tation; there is no separation into dependent and independent variables; the original set of correlated variables is transformed into a smaller set of uncorrelated variables called the principal components. Often used as the first step in a factor analysis.

PRION A virus-like particle, an infectious protein, to which several so-called slow virus diseases are attributed, including Creutzfeldt-Jakob disease and kuru in humans, bovine spongiform encephalopathy in cattle, and scrapie in sheep. Unlike viruses and bacteria, prions do not contain DNA or RNA. The word was coined in 1982 by the American neurologist Stanley B. Prusiner, from proteinaceous infectious particles, reversing the order of the vowels. Prusiner received the Nobel Prize in 1997 for the discovery of prions.

PRIOR ODDS The odds of disease before knowing the symptom. See Bayes’ theorem.

PRIOR PROBABILITY Probability calculated or estimated from past data, theory, and judgment, before a study is analyzed. See Bayes’ theorem.

PRIVACY The state of being undisturbed or free from public attention. The rules, regula- tions, and laws governing privacy and access to health-related information vary and change frequently. Privacy and confidentiality are protected by public interest groups and in some nations by privacy commissioners; the safeguards can affect epidemio- logical research requiring access to personal, private information. The concepts are increasingly important in situations where epidemiologists require access to data in medical and other records.186,319 See also informed consent.

PROBABILITY DENSITY For a continuous variable, the function that gives the rate at which its probability distribution is increasing. Not to be confused with frequency distribution.

PROBABILITY DISTRIBUTION For a discrete random variable, the function that gives the probabilities that the variable equals each of a sequence of possible values. Examples include the binomial and Poisson distributions. For a continuous random variable, the function that gives the probability that the variable is less than or equal to any given value; but the term is often used synonymously with the probability density function.

PROBABILITY OF CAUSATION For a given case, the probability that exposure played a role in disease occurrence.12 Although it is sometimes equated with the attributable fraction, the probability of causation can be much greater than that fraction. To illustrate, suppose 100 patients given an incorrect treatment each lived 8 years but would have lived 10 years each had they not received the incorrect treatment. Then the probability of causation of death by the incorrect treatment is 100%. After 10 years of follow- up, the total person-time at risk of death was 100(8) = 800; but if the incorrect treatment had not been given, it would have been 100(10) = 1000. Therefore the death rate over the 10 years was 100/800 = 0.125/year, but it would have been only 100/1000 had the incorrect treatment not been given. Thus the attributable fraction for the death rate was (0.125 – 0.100)/0.125 = 0.20 or 20%, which is far less than the probability of causation. In realistic examples the difference is less dramatic, but it still may be huge.

PROBABILITY SAMPLE (Syn: random sample) See sample.

PROBABILITY THEORY The branch of mathematics dealing with the purely logical properties of probability. Its theorems underlie most statistical methods.

PROBAND See propositus.

PROBLEM-ORIENTED MEDICAL RECORD (POMR) A medical record in which the patient’s history, physical findings, laboratory results, etc., are organized to give a cumulative record of problems (e.g., hemoptysis, rather than disease, such as pneumonia). The record includes subjective, objective, and significant negative information; discus- sions and conclusions; and diagnostic and treatment plans with respect to each prob- lem. This type of record, which was developed by Lawrence Weed,320 contrasts with the traditional medical record, which is less formally organized, usually recording all information from each source (history, physical, and laboratory findings) together without regard to the problems the information describes.

Since the problems may not be described in terms of conventional disease labels, their classification and counting for epidemiological purposes are sometimes difficult. The International Classification of Health Problems in Primary Care (ICHPPC) is an attempt to overcome this difficulty.

PROCARCINOGEN The precursor of an active carcinogen. The procarcinogen itself is not usually carcinogenic but is converted to the active carcinogen after it has been metabolized by “xenobiotic metabolizing enzymes” such as cytochrome P450, depend- ent monooxygenases, glutathione S-transferases, sulfotransferases, and others. To elicit detrimental effects, the great majority of human chemical carcinogens require meta- bolic activation.219,309 See also carcinogen.

PROCATARCTIC CAUSE A term used by epidemiologists of the late nineteenth and early twentieth centuries to describe predisposing causes associated with habits of life.

PRODUCT LIMIT METHOD See Kaplan-Meier estimate.

PROFESSIONAL ACTIVITY STUDY (PAS) The hospital discharge abstract system, which covers many acute short-stay hospitals in the United States. It provides regularly published statistical tables arranged according to hospital service, diagnostic category, etc., giving details on diagnostic and therapeutic procedures, length of stay, and outcome.

PROFILE PLOT (Syn: barycentric coordinates) A graphical method of data presentation used when several categories add to 100%; it permits the categories to be plotted on a plane surface using coordinates running inward at right angles from each side of an equilateral polyhedron.


  1. A (formal) set of procedures to conduct an activity, e.g., control of malaria.
  2. An ordered list of instructions directing a computer to carry out a desired sequence
    of operations. The objective is normally the solution of a problem.

PROGRAM EVALUATION AND REVIEW TECHNIQUES (PERT) A work-scheduling method that uses algorithms and also enunciates general principles of procedure for allocating resources. It calls for the listing of specific tasks to be completed and the resources—personnel, equipment, supplies, and other items—that will be needed, along with their costs; a time chart indicating when each component task is to begin and end; an enumeration of interim accomplishment levels during that period; and a specification of times for interim review of the progress of the plan.

PROGRAMMING The process whereby a stimulus or insult at a sensitive period of development has lasting effects on the structure or function of the body. It has other meanings in other areas of science. It is no longer recommended to describe the developmental origins of adult disease.45 See developmental origins hypothesis.

PROGRAM REVIEW An evaluative study of a specific health program operating in a specific setting that is performed to provide a basis for decisions concerning the operation of the program.

PROGRAM TRIAL An experimental or quasi-experimental evaluative study of a (health) program.

PROLECTIVE Pertaining to data collected by planning in advance. A research study may use data collected for the study purposes or retrolective data. Two of many imaginative terms coined by Alvan R. Feinstein (1925–2001),321,322 aiming to be more precise than the common terms prospective and retrospective. Rarely used. See also directionality.

PROPENSITY SCORE Conditional probability of being treated given a certain set of measured covariates. It can be used to control confounding through matching, strati- fication, regression adjustment, or a combination of these methods, and it can be used along with other covariate adjustments.323

PROPERTIES OF A CAUSE As described by David Hume in 1739, they are association (cause and effect occur together), time order (causes precede effects), and connection or direction.10,71 See also causal criteria; causality.

PROPHYLAXIS The preventive management of disease in individuals and populations. See also chemoprophylaxis.

PROPORTION A type of ratio in which the numerator is included in the denominator. The ratio of a part to the whole, expressed as a “decimal fraction” (e.g., 0.2), as a “common fraction” (1/5), or as a percentage (20%). By definition, a proportion (p) must be in the range (decimal) 0.0 p 1.0. Since numerator and denominator have the same dimension, any dimensional contents cancel out, and a proportion is a dimensionless quantity. Where numerator and denominator are based on counts rather than measurements, the originals are also dimensionless, although it should be understood that proportions can be used for measured quantities (e.g., the skin area of the lower limb is x percent of the total skin area) as well as for counts (e.g., 15% of the population died). A prevalence is a count-based proportion. The nondimensionality of a proportion, and its range limitations, do not necessarily apply to other kinds of ratios, of which “proportion” is a subset. See also rate; ratio.

PROPORTIONAL MORTALITY-ODDS RATIO The odds of deaths from a specified condition in a defined population divided by the odds of deaths expected from this

Proportional mortality ratio 198

condition in a standard population, usually expressed either on an age-sex-specific basis or sex-specific after age adjustment. Often preferred to the proportional mortality ratio on the grounds that it tends to better approximate the ratio of mortality rates in the two populations.

PROPORTIONAL MORTALITY RATIO The proportion of observed deaths from a specified condition in a defined population divided by the proportion of deaths expected from this condition in a standard population, usually expressed either on an age-sex- specific basis or sex-specific after age adjustment. Unlike the standardized mortality ratio, it does not require data on the age composition of the population but only on the deaths. The acronym PMR is preferably avoided because the same initial letters can stand for perinatal mortality rate.

PROPOSITUS (Syn: proband) The family member who first draws attention to a (genetic) pedigree of a given trait. The index case in a genetic study.

PROSPECTIVE STUDY See cohort study.

PROTOCOL The plan, or set of steps, to be followed in a study or investigation or in an intervention program. See also algorithm, clinical.

PROTOPATHIC BIAS A type of bias that can occur if the first symptoms of the outcome of interest are the reasons for using the treatment under study.321 See also confounding by indication.

PROXIMAL DETERMINANT See determinant, proximal (proximate).

PSEUDO-LONGITUDINAL STUDY If a population is randomly sampled, for example annually, it is possible to identify a sequence of subsamples with age ranges increasing in step with the dates of the samples. The sequence of subsamples resembles a regularly measured cohort except that it consists of a different sample at each sampling occasion, which may overlap little if at all.

PSYCHOSOCIAL EPIDEMIOLOGY See social epidemiology.


  1. The result of the tendency of authors to submit, organizations to encourage, reviewers to approve, and editors to publish articles containing “positive” findings (e.g., a gene- disease association), especially “new” results, in contrast to findings or reports that do not report statistically significant or “positive” results.
  2. Tendency of authors to preferentially include in their study reports findings that conform to their preconceived notions or outcomes preferred by their institution or sponsor.

Publication bias distorts available scientific evidence on a wide range of issues. It can be a particularly important source of bias in meta-analysis.106,280 See also epistemic communities; knowledge construction; interpretive bias; repression bias; suppression bias; scientific misconduct.


  1. One of the efforts organized by society to protect, promote, and restore the people’s health.
  2. The Acheson Report324 offered this definition: “The science and art of preventing disease, prolonging life, and promoting health through organized efforts of society.”
  3. The combination of sciences, skills, and beliefs that is directed to the maintenance and
    improvement of the health of all the people through collective or social actions.

The programs, services, and institutions involved emphasize the prevention of disease and the health needs of the population as a whole. Public health activities change with changing technology and social values, but the goals remain the same: to reduce the amount of disease, premature death and disease-produced discomfort and disability in the population. Public health is thus a social institution, a discipline, and a practice.1,85,225,313,325

PUBLIC HEALTH IMPACT ASSESSMENT An analysis, evaluation, and assessment of the consequences and implications for public health of specific social or environmental initiatives or processes (e.g., construction of a power plant, housing developments and roads, legal status of immigrants). A combination of procedures, methods, and tools by which a policy, program, or project may be judged as to its potential effects on the health of a population and the distribution of those effects within the population. Considered as a major opportunity to integrate health into all policies, PHIA aims to influence the decision-making process, addressing all determinants of health, tackling inequities, and providing a new impetus for participation and empowerment in health.70,326

PUBLIC HEALTH MEDICINE The practice of public health by physicians. See social medicine.

PUNCH CARD A card on which data were stored by means of holes punched in specified positions. The position of the hole was the means of identifying the value of a variable. Punch cards were sorted mechanically or electrically to process and analyze data. They became obsolete in many areas by the mid-1980s, following the advent of computerized data processing. Hollerith cards were a commonly used variety of punch cards.

P VALUE (probability)

  1. The probability that a test statistic would be as extreme as observed or more extreme
    if the null hypothesis were true. The letter P stands for this probability. It is usually close to the probability that the difference observed or greater could have occurred by chance alone, i.e., under the null hypothesis. Investigators may arbitrarily set their own significance levels, but in most biomedical and epidemiological work, a study result whose P value is less than 5% (P
    < 0.05) or 1% (P < 0.01) is considered sufficiently unlikely to have occurred by chance to justify the designation “statistically significant.”
  2. More generally, the probability that a test statistic would be as extreme as or more extreme than observed if the test hypothesis were true. It is usually close to the probability that the deviation between what was observed and the hypothesized (test) value for the difference, or a greater deviation, could have occurred by chance alone. See also statistical significance.

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