Case Control Studies: Chapter 8 Epidemiology

What is the difference between an ''incident case'' and ''prevalent case''?

An incident case is one of onset during the period of observation. The onset of a prevalent case preceded the period of observation.

What does an odds ratio of 0.5 represent?

An odds ratio of 0.5 represents a negative association in which exposed individuals have half the risk as non-exposed individuals.

What does an odds ratio of 1 represent?

An odds ratio of 1 represents no association between the exposure and disease.

What does an odds ratio of 2 represent?

An odds ratio of 2 represents a positive association in which exposed individuals have twice the risk of disease as the nonexposed individuals.

Identify sources of controls for case-control studies.

Controls may be derived from random samples from the population, for the capturement area from the clinics and hospitals that were the source of cases, from among hospitalized individuals with ailments that are unrelated to the disease being studied, and so on.

What do the symbols A and B represent in the 2-by-2 table notation?

In this book's notation, A represents the number of cases and B represents the number of non-cases in the study.

Identify sources of cases in case-control studies.

We can find cases for case-control studies in medical facilities, disease registries, surveillance systems, computerized medical record systems, and death certificates.

If cases were derived from a hospital, what is the best population-based source of controls?

The best source of population-based controls for hospitalized cases is the capture-ment area for that hospital.

What distinguishes case-control studies from cohort studies?

The main distinction between cohort studies and case-control studies is the way in which they select subjects for study. Cohort studies select healthy individuals and go on to determine the incidence of disease. Case-control studies select diseased individuals (cases) and non-diseased individuals (controls) and determine the occurrence of prior exposures.

What is the primary function of the control series in a case-control study?

The primary function of the control series in a case-control study is to reflect the relative frequency of the exposure in the source population.

What do the subscripts 1 and 0 represent in the 2-by-2 table notation used in the text?

The subscript 1 denotes ''exposed'' while the subscript 0 denotes ''non-exposed.''

What are the two most common types of observational studies in epidemiology?

The two most common types of analytic studies in observational epidemiology are cohort studies and case-control studies.

What is a nested case-control study?

A nested case-control study identifies cases as part of a cohort study. It then samples non-cases from the study cohort to serve as controls.

*DO 8 EXERCISES

*DO 8 EXERCISES

Sources of cases

- Population surveillance - Hospitals and clinics - Death certificates - Administrative health records - Pharmacy records

Fill in the blank: Maximum statistical efficiency for a given total sample size in a case-control study is achieved when we select _____ control for each case.

1

Fill in the blank: Increases in statistical power can be gained by selecting multiple controls per case in a case-control study. However, very little gain is achieved once we select more than __ controls per case.

4

What is a ''case definition''?

A case definition is the set of uniform criteria by which to decide whether an individual should be classified as a case in an epidemiologic investigation.

Multiple choice: The odds ratio from a case-control study is stochastically equiva- lent to a(n): (a) incidence (b) prevalence (c) rate ratio (d) rate difference.

C

Multiple choice: When there is no association between the exposure and disease, the odds ratio is equal to (a) -1 (b) 0 (c) 1 (d) 100.

C

Why are case-control studies efficient for studying rare diseases?

Case-control studies are statistically efficient when studying rare diseases because they use a small fraction of the non-cases from the source population to estimate the frequency of exposure in the underlying population.

What measure of effect is available to case-control studies?

Case-control studies can calculate odds ratio.

Why are case-control studies unable to determine the incidence of outcomes?

Case-control studies cannot determine the incidence or prevalence of disease because they do not determine number of individuals or number of individuals at risk in the population or population subgroups.

This statistic is also called the cross-product ratio.

Odds Ratio

Prevalent cases

Onset of disease before study interview

Incident cases

Onset of disease during study interval

Fill in the blank: Odds ratios from case-control studies are stochastically equivalent ________ ________ in the underlying source population.

Relative risks (or rate ratios).

Sources of Exposure Information

• Interview • Medical records • Birth & death certificates • Employment records • Environmental records • Biological specimens