Inam, S N

Lobo, M A

Quresh, A F

English

eCommons@AKU

eCommons@AKUCommunity Health Sciences Department of Community Health SciencesNovember 1993Practical epidemiology and biostatistics in researchepidemiologic glossaryA F. QureshAga Khan UniversityM A. LoboAga Khan UniversityS N. InamAga Khan UniversityFollow this and additional works at: https://ecommons.aku.edu/pakistan_fhs_mc_chs_chsRecommended CitationQuresh, A. F., Lobo, M. A., Inam, S. N. (1993). Practical epidemiology and biostatistics in research epidemiologic glossary. Journal ofPakistan Medical Association, 43(11), 245-247.Available at: https://ecommons.aku.edu/pakistan_fhs_mc_chs_chs/533Practical Epidemiology and Biostatistics in ResearchEpidemiologic glossaryPages with reference to book, From 245 To 247 Asma Fozia Qureshi, Melvyn A. Lobo, S.N. Bazmi Inam ( Department of Community Health Sciences, Aga KhanUniversity, Karachi. ) The glossary presents terms (in alphabetical order) used in scientific literature and Commonlyencountered in articles, books and journals.Association (Syn: relationship, correlation)Statistical dependence between two or more events or variables. The association is described aspositive when the occurrence of higher values of a variable is associated with the occurrence. of highervalues of another variable. In a negative association the occurrence of higher values of one variable isassociated with lower values of the other variable.Attack Rate (Syn: cumulative incidence)Used in studies of infectious diseases. It is calculated by dividing the total number of persons that gotsick by the total number that were exposed to the risk (e.g,. number of people who developed diarrhoeaand vomiting after eating ice cream at a party for a 300 people).Attributable Risk (AR)Measures the excess risk of disease ctue to exposure to a risk factor. It is the absolute difference inincidence rates of disease in the exposed and unexposed populations. If there is no difference in the twoincidence rates the AR will be zero. If there is causal association between the risk factor and the diseasethe AR will be more than zero, and its value will indicate the anticipated quantitative impact of apreventive intervention. This measure of risk can be calculated from cohort studies.BIASDeviation of results from the truth. Any trend in the collection, analysis or interpretation of data thatcan lead to conclusions that are systematically different from the truth. “Selection Bias” results whenthe selection criteria for subjects differ between compared groups in relation to the exposure oroutcome of interest, leading to non-comparability. “Recall Bias” may arise because individuals exposedto a particular hazard or disease remember their experiences differently or report subsequent eventswith a different degree of completeness or accuracy than those not exposed.Case Control Study (Syn: retrospective study)A study in which cases (persons with the disease) are compared with appropriate controls (personswithout the disease) on the basis of exposure to aetiologic factors in the past histories. This is done todetermine which of their past exposures/experiences is relevant to the disease. The study starts after theonset of disease and looks back to possible causal factors.Case: Fatality RateThe number of cases dying from a disease divided by the total cases during a specified interval of time.Cause-specific RatesThe number of deaths or illness due to a specific disease divided by the total population.Cohort Study (Syn: prospective study, follow-up study)A study in which individuals are selected on the basis of being exposed or non-exposed to a potentialrisk factor. These individuals are followed up for a period of time to compare the development orincidence of disease in the two groups.Confidence Interval (Syn: confidence limits)The range likely to include the true value of the parameter of concern at a stated level of probability,such that the value of the parameter is expected to be included in a given percentage of repeatedsampling estimation. It provides a useful device for arguing that the sample results are representative ofthe population from which the sample was selected. For example, 95% confidence limits signify a 5%chance of error in representativeness of the sample result.ConfoundingA situation in which a measure of the effect of an exposure on outcome is distorted because of theassociation of exposure with other factors (confounders) that can influence the outcome.Cross-sectional Study (Syn: prevalence study)A study in which the study population is selected independent of their disease and exposure statuswhich are then determined at the time of the study. It provides a ‘snap-shot’ of the population understudy at a point in time. As temporal sequence between cause and effect generally cannot bedetermined, conclusions regarding causality cannot be drawn. Disease prevalence rather than incidenceis records from this type of study.EndemicThe usual prevalence or constant presence of a disease or infectious agent in a given geographic area orpopulation group.EpidemicThe occurrence in a community or region of a group of cases of an illness (or an outbreak) clearly inexcess of normal expectancy. An epidemic occurring over a very wide area and usually affecting a largeproportion of the population within a stated period of time or age. Also a non-technical termencompassing a variety of measures of the probability of an (generally un-favourable) outcome.Incidence RateThe number of net cases of a disease that develop in a population at risk during a specified period oftime; this is also called “cumulative incidence”. “Incidence density” is the number of new cases thatdevelop in a population divided by the sum of each individual’s time at risk; i.e., not all persons at riskare followed for equal periods of time, therefore, the time that each person is observed is added to givethe total “person-time” of observation which is often used as the denominator.Incubation PeriodThe interval between exposure to infection and the appearance of the first symptom.Infective PeriodThe period of time that an individual is capable of transmitting disease with or without contact.Mortality RateThe Incidence rate of deathOdds Ratio (OR)The measure of association generally used in a case-control study. It is a ratio of the odds of exposureamong the cases to the odds of exposure among the controls. Generally the odds ratio is an appropriateestimate of the relative risk.Population Attributable RateA measure of the amount of disease associated with an exposure within a population. It is the differencebetween the incidence of disease in the entire population and that among the non-exposed.Prevalence RateThe number of existing cases divided by the number of persons (bothil and well) in a population at aspecified point in time.P ValueA measure of the consistency of the data with the null hypothesis, i.e., it means was the probability thatthe difference between two sets of values are due to chance. Conventionally a p value of less than 0.05is considered significant, indicating that the difference seen has a less than 5% probability of havingoccurred by chance.RateThe number of events (e.g. death or disease frequency) in a specified period divided by the populationat risk of the events in the same time period.Relative RiskMeasures the risk of disease occurrence. It is calculated by dividing the incidence of disease in theexposed by that in the unexposed. This measure of association is calculated from prospective or cohortstudies.RiskA probability that an individual will become ill within a stated period of time or age. Also a non-technical term encompassing a variety of measures of the probability of an (generally unfavourable)outcome.ScreeningScreening is a method for presumptive identification of unrecognized disease or defect by use of teststhat can be applied rapidly and economically. The purpose is to distinguish among apparently normalpeople those who probably have a disease from those who probably do not. Screening tests are notdiagnostic; additional tests and examination are needed to establish definitive diagnosis.Variable:Any class of measurement (e.g., sex, age, haemoglobin, etc) on which individual observations are madeis called a variable. Discrete variables are those for which the possible values are quite distinct andseparated such as grades of students performance in a class. Continuous variables are those which canassume an uninterrupted or continuous range of values such as height, weight, blood pressure, etc.Studies often look at the relationship between two variables, and how one variable (X) changes withanother (Y). The variable under study i.e. X is the "outcome/dependant’ variable whereas Y is the“predictor/independent" variable as it predicts the values that variable X will assume for any givenvalue of Y.Standard Mortality Ratio (SMR)“Standardization” is a procedure of adjustment used on crude rates which are influenced by differencesin population structure (such as age or sex). The SMR is the ratio of the number of observed deaths in apopulation divided by the expected number of deaths in that population, commonly multiplied by 100.The denominator (i.e., the expected number of deaths) is calculated by applying age-specific rates of astandard population to the population of interest. Adjustment thus allows comparisons of rates of twopopulations.References 1. Hennekens, C.H., Buring, J.E. Epidemiology in medicine. S.L. Myrent (Ed). 1st edition, Boston,Little Brown and Co., 1987.2. Last, 3M. (Ed). A dictionary of epidemiology, (2nd ed), Oxford, Oxford University Preas, 19883. Norman, G.R., Streiner, D.L. PDQ statistics, Asian Edition, Singapore, Maulygraphic Publishers Pte.Ltd., 1989.

Practical epidemiology and biostatistics in research epidemiologic glossary

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