Chapter 6: Key Terms

Ecologic Comparison Study
Involves an assessment of the association between exposure rates and disease rates during the same time period.
Ecologic Correlation
An association between two variables measured at the group level.
Ecologic Fallacy
An erroneous inference that may occur because an association observed between variables on an aggregate level does not necessarily represent or reflect the association that exists at an individual level.
Case-Control Study
A study in which subjects are defined on the basis of the presence or absence of an outcome of interest. The cases are those individuals who have the outcome or disease of interest, whereas the controls do not. Can examine only a single outcome or a limited set of outcomes because having a specific outcome is the criterion for being included in the case group.
Matched Case-Control Study
Study in which the cases and controls have been matched according to one or more criteria such as sex, age, race, or other variables.
Odds Ratio (OR)
A measure of the association between frequency of exposure and frequency of outcome used in case-control studies. Called an indirect measure of risk because incidence rates have not been used; instead, the risk of an outcome associated with an exposure is estimated by calculating the odds of exposure among the cases and controls.
-An odds ratio of more than 1.0 suggests a positive association between the exposure and disease or other outcome.
-An OR less than 1.0 indicates that the exposure might be a protective factor.
-When OR=1, there is no association between exposure and outcome.
Retrospective Approach
This method is used in case-control studies. The investigator starts with subjects who already have a disease and queries them about previous exposures that may have led to the outcome under study.
Cohort Study
Defined as a population group, or subset thereof (distinguished by a common characteristic), that is followed over a period of time. Able to evaluate many different outcomes (causes of death) but few exposures because exposure is the criterion used to select subjects into a cohort study.
Difference in Risk
-Attributable Risk
-Population Risk Difference
Relative Risk (RR)
The measure of association used in cohort studies. The ratio of the incidence rate of a disease or health outcome in an exposed group to the incidence rate of the disease or condition in a nonexposed group.
RR=(Incidence rate in the exposed)/(Incidence rate in the nonexposed)
-A RR of 1.0 implies that the risk (rate) of disease among the exposed is not different from the risk of disease among the nonexposed.
-A RR >2.0 implies that the risk is more than twice as high among the exposed as among the nonexposed.
-If RR is sometimes called a protective effect.
Crossover Design
Participants may be switched between treatment groups (e.g., members of treatment group A are transferred to treatment group B, or vice versa).
Prophylactic Trial
Designed to test preventive measures.
Therapeutic Trial
Evaluate new treatment methods.
Hawthorne Effect
Refers to participants’ behavioral changes as a result of their knowledge of being in a study.
Program Evaulation
The determination of whether the program meets stated goals and is justified economically.
Quasi-Experimental Study
A type of research in which the investigator manipulates the study factor but does not assign individual subject randomly to the exposed and nonexposed groups.
Obtaining information about exposures that occurred in the past. Start with subjects who already have a disease and ask them about previous exposures that may have led to the outcome under study.
Ecologic Study
The group is the unit of analysis. A study in which the units of analysis are populations or groups of people rather than individuals. Information about both exposures (explanatory variables) and outcomes is collected at the group level.
Prospective Cohort Study
Subjects are classified according to their exposure to a factor of interest and then are observed over time to document the occurrence of new cases (incidence) of disease or other health events.
Retrospective Cohort Study
Makes use of historical data to determine exposure level at some baseline in the past; follow-up for subsequent occurrences of disease between baseline and the present is performed.
Historical Prospective Cohort Study
Combines retrospective and prospective approaches.
Attributable Risk
A type of difference measure of association. Refers to the difference between the incidence rate of a disease in the exposed group and the incidence rate in the nonexposed group.
AR=(Incidence rate in the exposed group)-(Incidence rate in the nonexposed group)
Population Risk Difference
Provides an indication of the benefit to the population derived by modifying a risk factor.
PRD=(Incidence in the total population)-(Incidence in the nonexposed segment)
Intervention Study
An investigation involving intentional change in come aspect of the status of the subjects, e.g., introduction of a preventive or therapeutic regimen or an intervention designed to test a hypothesized relationship.
Randomized Controlled Trial (RCT)
An epidemiological experiment in which subjects in a population are randomly allocated into groups, usually called study and control groups, to receive or not to receive an experimental preventive or therapeutic procedure, maneuver, or intervention.The results are assessed by rigorous comparison of rates of disease, death, recovery, or other appropriate outcome in the study and control groups.
Clinical Trial
Refers to a research activity that involves the administration of a test regimen to humans to evaluate its efficacy and safety.
Community Intervention
An intervention designed for the purpose of educational and behavioral changes at the population level.
External Validity
Refers to one’s ability to generalize from the results of the study to an external population.
Sampling Error
A type of error that arises when values (statistics) obtained for a sample differ from the values (parameters) of the parent population.
Internal Validity
Refers to the degree to which the study has used methodologically sound procedures.
Systematic deviation of results or inferences from truth. Processes leading to such deviation. An error in the conception and design of a study-or in the collection, analysis, interpretation, reporting, publication, or review of data-leading to results or conclusions that are systematically (as opposed to randomly) different from the truth.
Recall Bias
Particularly relevant to case-control studies. Refers to the fact that cases (subjects who participate in the study) may remember an exposure more clearly than controls. Consequence is a reduction in the reliability of exposure information gathered from control groups.
Selection Bias
Distortions that result from procedures used to select subjects and from factors that influence participation in the study. A distortion in the estimate of the effect due to the manner in which subjects are selected for the study.

Error that occurs when the relationship between exposure and disease is different for those who participate in a study versus those who would be theoretically eligible for the study but do not participate.

Healthy Worker Effect
A type of selection bias. Refers to the observation that employed populations tend to have lower mortality experience than the general population. May have an impact on occupational mortality studies.
-People whose life expectancy are shortened by a disease are less likely to be employed than healthy persons.

Consequence would be a reduced (or attenuated) measure of effect (e.g., odds ratio or relative risk) for an exposure that increases morbidity or mortality.

Denotes the distortion of a measure of the effect of an exposure on an outcome due to the association of the exposure with other factors that influence the occurrence of the outcome.
Aggregate Measures
Provide an overall measurement for the level, e.g., group or population, being studied; group measurements.