ActivEpi Flashcards HOSA Epidemiology

Epidemiology

important factors or variables that influence a health outcome of interest.

Exposure Variable

An independent variable or predictor of primary interest in an epidemiologic study; a variable that an investigator wants to assess as to whether or not it predicts a health outcome.

Covariate

A variable that is measured or observed in a study; often considered as a control variable in the data analysis.

Health Outcome

The possible health-related states or events of interest; the dependent variable of interest; in epidemiology, usually the disease variable being studied, although the outcome of interest may alternatively be mortality status, recovery from illness, or any health condition other than a disease.

Controlling

Adjusting for the influence of variables other than the exposure variable that have potential influence on the health outcome.

Disease Frequency

How often a disease/ health outcome occurs in a given study.

Association

The relationship between two or more variables; in epidemiology, an association is quantified using a measure of effect, e.g., an odds ratio, a risk ratio; a popular statistical measure of association is a correlation coefficient.

Bias

The presence of systematic error; a flaw in the study design, the methods of data collection, or the methods of data analysis that may lead to spurious conclusions about a exposure-disease relationship.

3 General Sources of Bias

the selection of study subjects, incorrect information gathered on study subjects and the failure to adjust for variables other than the exposure, commonly called confounders.

Causation

Relationship between a cause and an effect, i.e., knowledge or measurement of one variable (i.e., the cause) leads to knowledge of another variable (i.e., the effect).

Casual Inferences

Drawing conclusion about causality involving one variable as a "cause" (i.e., determinant) of another variable "the effect", (usually a health outcome in epidemiology) where the latter is usually a health outcome.

Meta Analysis

Statistical methods for contrasting and combining results from different studies in the hope of identifying patterns among study results, sources of disagreement among those results, or other interesting relationships that may come to light in the context of multiple studies.

Dose-Response Effect

A relationship between two variables, say Y and X, such that Y increases or decreases as X increases.

Counterfactual

An event, condition, or feature of a study that cannot actually be experienced by a person or group at the same time that the same event, condition or study feature has not been experienced by the same person or group

Counterfactual Study Design

A follow-up (i.e., cohort) study design to assess the relationship between a binary exposure variable (E)( same thing) and a health outcome variable (D) in which each person in the study is assigned to both exposure groups at the start of follow-up.

Casual Contrast

A measure of effect used to evaluate the extent to which E is causally predictive of D is given by R_C = p_1C/ p_0C, where the proportion of persons who develop the disease when exposed is denoted by p_1C and the proportion of persons who develop the disease when unexposed is denoted by p_0C,

Predictor

Synonym for independent variable.

Cohort

A group of persons who share a common attribute, such as birth in a particular year, or residence in a particular town, who are followed over time; an observational study design involving such a group is called a cohort study.

Case-Control

A study that starts with persons who either have a health outcome (i.e., the cases) or do not have the health outcome (i.e., the controls), and looks back in time to determine prior exposure status

Cross-Sectional

A basic observational study design in which all variables studied are observed at a single point or a very short period in time.

Sampling plan

The plan for determining how study subjects are to be sampled in the study.

Measure of Disease Frequency

A measure of how often a disease or other health outcome occurs in a population.

Measure of Effect

A quantity that expresses the strength of association between two variables,

Selection Bias

Systematic error that results from the way subjects are selected into the study and/or because there are selective losses of subjects prior to data analysis.

Information Bias

Systematic error resulting from incorrect information obtained on one or more variables measures in a study. If all study variables of interest are categorical (e.g., dichotomous),

Causal diagram

A graphical picture that identifies the variables being considered in a question about causation using lines

Directed Acyclic Graph

A causal diagram that connects variables being considered in an epidemiologic study using lines and/or arrows. Such a diagram is acyclic because the arrows or lines do not start and end at the same variable.

Variability

A measure of the amount that the values of a variable differ from one another; the most common measure of variability is variance, or its square root, the standard deviation

Mathematical Modeling

An option available at the analysis stage of a study for the control of extraneous variables in which a mathematical formula or function is used to describe the relationship between a health outcome variable and one or more explanatory variables (e.g., exposures confounders and effect modifiers).

Binary

A variable that takes on one of two possible values. Usually, though not always, coded as 0 or 1.

Outbreak

An occurrence of disease greater than expected at a particular time and place. It may affect a small and localized group or impact thousands of people across an entire continent.

Hypothesis-generating

A type of research effort/study that is primarily intended to spot possible leads worthy of further research. In epidemiology, the variables being considered in a hypothesis-generating study are typically treated as exposure variables rather than control variables.

Cohort Study

An observational study in which subjects are sampled based on presence (exposed) or absence (unexposed) of an exposure variable of interest; these subjects are followed over time for the development of a health outcome of interest; often used as a synonym for follow-up study.

Conceptual hypothesis

A statement or conjuncture about a possible exposure-disease relationship that is based on insight obtained from research literature or conceptual theory.

Operational hypothesis

A statement about a possible exposure-disease relationship that translates a conceptual hypothesis into a testable form involving measurable variables and a clearly specified study plan.

Randomization

An allocation procedure for assigning subjects to exposure/study groups by chance that attempts to make groups comparable with respect to other predictors of the health outcome.

Experimental

A study in which randomization is used to allocate subjects to exposure/treatment/study groups.

Observational

A study that does not involve randomization; the investigator monitors but does not influence the exposure status of individual subjects and their subsequent health outcome status.

Directionality

A design option that answers the question, "Which did you observe first, the exposure or the disease?"

Timing

A design option that answers the question, "has the information being studied already occurred before the study actually began?"

Retrospective

A study in which the data are derived from past experience; both the study factor and the health outcome occur before the onset of the study.

Prospective

A study in which both the study factor and the health outcome occur after the study onset; new data are used.

Efficacy

The effect that a treatment or other clinical procedure has on the well-being of individuals.

Backwards

The directionality of a study in which the health outcome is observed before the exposure variable is observed; the study proceeds "backwards" over time; a case-control study always has backwards directionality.

Forwards (directionality)

The directionality of a study in which the health outcome is observed after (in time) the exposure variable is observed

Non-directional

A study in which the health outcome and the exposure variable are observed at the same time; the study is essentially a snapshot time, i.e., a survey; a cross-sectional study is always non-directional.

Measurement error

Incorrect measure or observation of a variable of interest; can lead to information bias or misclassification bias.

Clinical Trial

An epidemiologic study design that most closely resembles a laboratory experiment; the major objective is to test the efficacy of a therapeutic or preventive intervention

Blinding

• A feature of a clinical trial in which the patient/subject and/or the investigator is unaware of the treatment assigned.

Intention-to-treat analysis

Analysis of the results of a clinical trial that is based on the initial treatment assignment regardless of whether or not subjects completed the full course of treatment, i.e., "you analyze what you randomize"

Community intervention study

A study/investigation in which communities or groups are randomized to different treatment arms; similar to a clinical trial, except that randomization is done on a community/group basis rather than on an individual basis.

Efficiency (of a health care intervention)

A measure of the health status improvement resulting from a given amount of resources expended; typically determined via a cost-effectiveness analysis or cost-utility analysis.

Nested Case-Control

A hybrid design that is a variation of the case-cohort design in which controls are sampled by density sampling; that is, controls are matched to cases on the time of the case diagnosis.

Ecologic

Observational studies in which the unit of analysis is a group, often defined geographically, such as a census tract, a state, or a country; primary criticism: there is no data on individuals.

Observational study design

A study design that does not involve randomization; the investigator monitors but does not influence the exposure status of individual subjects and their subsequent health outcome status.

Rare Disease

A disease that occurs so infrequently that the risk for any study subject is approximately zero.

Latencies

Time between exposure to as risk factor and subsequent development of clinical manifestations of a particular disease.

Proportional

An observational study design that only includes observations on cases without information about the candidate population at risk for developing the health outcome

Cohort Study Characteristics

• Always a follow-up study with forward directionality.

• Study can be prospective or retrospective. 

• Least prone to bias when compared with other observational study designs.

• Forward directionality looks at cause before effect.

• Can study several diseases.

Disadvantages of a cohort study

• Often quite costly and time-consuming, particularly if prospective.

• Loss-to-follow-up may lead to bias.

• Bad design for studying rare diseases or diseases with long latencies.

Source Population

Alternatively called target population or study base. The population of restricted interest at risk for being a case and from which cases in one's study were derived

Case Control Study Characteristics

• Backwards

• Inexpensive

• Prone to bias

• retrospective or prospective in timing.

• providing sufficient numbers of cases for rare diseases with long latencies.

• allowing several exposures to be evaluated.

Disadvantages of case-control studies

• being susceptible to both selection and information bias.

• not allowing estimation of risk.

• not considering more than one disease.

• not feasible for rare exposures.

Cross-Sectional Study Characteristics

• Not expensive and quick to carry out.

• Representative sample.

• Useful for describing patterns of disease occurrence.

• Can study several diseases or exposures.

• Useful for generating hypotheses.

Disadvantages of a Cross-Sectional Study

• Can lead to bias because cases are survivors.

• Can't tell if exposure precedes disease.

• May under-represent short duration diseases such as the common cold or flu.

Hybrid designs

Observational studies that combine the elements of at least two basic designs, or extend the strategy of one basic design through repetition; two popular hybrid designs are the nested case-control study and the case-cohort study.

Case-crossover

which each case serves as its own control; evaluates the effect of brief exposures with transient effects on acute health outcomes

Nested Case-Control study

A hybrid design that is a variation of the case-cohort design in which controls are sampled by density sampling; that is, controls are matched to cases on the time of the case diagnosis.

Incomplete designs

Observational studies in which information is missing on one or more relevant factors, e.g., an ecologic study, a proportional mortality study or a proportional morbidity study.

Population-at-risk

The population or group that is eligible to have or develop a health outcome of interest, also called the candidate population, e.g., the disease-free population.

proportional morbidity study

living incident cases.

proportional mortality study

cases are deaths.

Incidence

New cases of a health outcome, e.g., disease, that occur in a defined population over a specified period of follow-up.

Prevalence

The proportion of persons in a defined population who have a particular health outcome, e.g., a disease, at a point or during a period of time; existing cases of a disease.

Risk

• The probability that an individual will experience an event of interest within a specified period of follow-up

Cumulative Incidence

• Gives the proportion of persons that develop a disease condition out of those candidates (e.g., disease-free) available for developing the disease condition.

• Can be computed using the formula for simple cumulative incidence CI = I/N, where N denotes the size of a disease-free fixed cohort at the start of follow-up and I denotes the number of cases of disease that develop over the follow-up period.

Simple Cumulative Incidence

A population-based estimate of risk that has the formula CI=I/N, where N denotes the size of a disease-free fixed cohort at the start of follow-up and I denotes the number of cases of a health outcome that develop over the follow-up period.

Fixed Cohort

A group (i.e., cohort) of subjects identified at some point of time and followed for a given period of detection of new cases. The cohort is "fixed" in the sense that no entries are permitted into the study after the onset of follow-up, although subsequent losses of subjects may occur for various reasons such as withdrawal, migration, and death.

Shifted cohort

A dynamic cohort for which subjects progressively enter the study at different calendar times, but whose follow-up time is shifted to start at the time of initial entry into the study.

Dynamic Cohort

A group of subjects that is continually changing, allowing for both the addition of new members and the loss of previously entered members during a period of follow-up.

Rate/Hazard

A measure of the rapidity with which health events such as new cases of disease or deaths occur; also called the hazard and incidence density.

Person-time

• The total amount of disease-free follow-up time experienced by all persons in a cohort that is being followed over a specified time period.

Stable dynamic population

A dynamic population that undergoes no major demographic shifts during the time period of study; the size of such a population is typically estimated by the size of the entire population based on census data available close to the time period of study.

Average rate

Number of new cases in (T₀,T₁) divided by PT

where T₀ and T₁ denote the starting and ending time points of follow-up, and PT denotes the amount of disease-free person-time accumulated during the interval from T₀ to T₁.

Measure of mortality frequency

Deaths; a measure of the frequency at which deaths occur.

Instantaneous rate

A measure of how rapidly cases of a disease or other health outcome are occurring at a point in time

Incidence rate

Synonym for rate and incidence density; a measure of the rapidity with which health events such as new cases of disease or deaths occur; also called hazard and incidence density.

Risk vs Rate

• Risk is always a proportion, whose values therefore lie between 0 and 1. Rate is not a proportion, and its values are always greater or equal to zero but have no upper bound.

• Risk is unit-free and will vary with length of the study period of follow-up. Rate is in units of time, and, when calculated as an average over a study period of follow-up, does not vary with the length of the study period.

mortality risk

simple cumulative incidence, namely, CI = I / N, can be used.

Mortality rate

average rate, namely, IR = I /PT.

Point prevalence

The proportion of persons in a defined population who have a particular health outcome, e.g., disease, at a point or during a period of time; existing cases of a disease at point in time.

Period prevalence

The proportion of persons in a defined population who have a particular health outcome, e.g., disease, during a period of time; existing cases of a disease during a period of time.

Disease-specific mortality

A risk or rate measure of the incidence of dying from a particular disease.

All-causes mortality

A risk or rate measure of the incidence of dying from all causes.

Case-fatality

A risk or rate measure of persons with a particular disease that die of that disease over a period of follow-up.

Mortality risk

A measure of an individual's probability of dying during a period of follow-up.

Mortality rate

A measure of the rapidity at which persons are dying in a cohort followed over time.

Age-adjusted

Summary (i.e., overall) rates, risks, or prevalence measures for comparing different populations in which underlying differences in the age distribution are removed. Also called age-standardized.

Crude Death Rate

Summary (i.e., overall) estimate of a death rate, risk, or prevalence measure that ignores the control of other variables.

Standard Population

A population used for computing age-adjusted rates that provides a common age distribution that allows the comparison of two or more groups with differing age distributions, e.g., if death rates for Arizona and Alaska are to be compared, the standard population would typically be the US population; can also be used for any kind of rate adjustment.