Retrospective assessment of the exposure 2011 1998 Exposed Unexposed Patient population Group with disease Group without disease Exposed Unexposed
Figure 4.3 Schematic of case control study design.
Case-Control Studies A case-control study starts with the identification of individuals with a disease or outcome of interest and a suitable control population without the disease or outcome of interest. The controls should represent a sample of the population from which the cases arose and who were at risk for the disease or outcome but did not develop it. The relationship between a particular attribute or exposure to the disease is retrospectively studied by comparing how the cases and controls differed in that exposure (Fig. 4.3).
Odds Ratio The measure of association for a case-control study is the odds ratio, which is the ratio of exposed cases to unexposed cases, divided by the ratio of exposed to unexposed controls (see the section on Presenting and Understanding the Results of Analytic Studies). If an entire population could be characterized by its exposure and disease status, the exposure odds ratio would be identical to the relative risk obtainable from a cohort study of the same population. Although the relative risk (RR) cannot be calculated directly from a case control study, it can be used as an estimate of the relative risk when the sample of cases and controls are representative of all people with or without the disease and when the disease being studied is uncommon. Attributable risk is not directly obtainable in a case-control study.
The advantages of case-control studies are that they are generally lower in cost and easier to conduct than other analytic studies. Case-control studies are most feasible for examining the association between a relatively common exposure and a relatively rare disease.
Disadvantages include greater potential for selection bias, recall bias, and misclassification bias.
Case-control studies may be especially prone to selection bias and recall bias. Investigators need to understand sampling issues around which cases and controls were selected for their study and how these may have affected exposure rates. Subtle issues, such as interviewer technique, may affect the likelihood that cases may recall or report exposures more readily than controls.
Cross-Sectional Studies Cross-sectional studies assess both the exposure and the outcome at the same point in time. Individuals are surveyed to provide a “snapshot” of health events in the population at a particular time. Cross-sectional studies are often called prevalence studies because the disease exists at the time of the study, and the longitudinal follow-up and disease duration are not known. Prevalence is the existing number of cases at a specific point in time.
Cross-sectional studies are often done to evaluate a diagnostic test. The value of the test (predictor) is compared with the outcome (disease). The results of these evaluations are often
Prevalence of disease is 50%
Sensitivity (a/a + c): 95/100 = 95% Specificity (d/b + d): 95/100 = 95% PPV (a/a + b): 95/100 = 95% NPV (d/c + d): 95/100 = 95%
Prevalence of disease is 9%
Sensitivity: 95/100 = 95% Specificity: 950/1000 = 95% PPV: 95/145 = 65.5% NPV: 950/955 = 99.5%
Figure 4.4 Comparison of sensitivity, specificity and predictive values when the prevalence of the disease varies.
presented as sensitivity and specificity. The sensitivity and specificity represent the characteristics of a given diagnostic test and do not vary by population characteristics. In contrast the negative and positive predictive values of a test do vary with the baseline characteristics of a population such as prevalence of a disease (Fig. 4.4).
Descriptive studies, case reports and case series, do not include comparison groups.
In a case report or case series, the characteristics of individuals who have a particular disease are described. A case report usually describes an unusual clinical scenario or procedure in a single patient, whereas a case series usually includes a larger group of patients with similar exposures or outcomes. Just because members of a case series share a particular characteristic, it cannot be assumed that there is a cause-and-effect relationship.