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M09.01.14 Case control study design
A case-control study is an observational study design commonly used to investigate the association between risk factors and outcomes. In such studies, researchers start by identifying cases (individuals with the disease) and controls (individuals without the disease) and look retrospectively at their exposure to potential risk factors. The primary measure used in case-control studies is the odds ratio (OR), which helps estimate the strength of the association between exposure to a risk factor and disease presence.
Odds Ratio (OR) Calculation
The odds ratio in a case-control study compares the odds of exposure among cases to the odds of exposure among controls. It can be calculated using the following formula:
Where:
- A = Number of cases exposed
- B = Number of controls exposed
- C = Number of cases not exposed
- D = Number of controls not exposed
Example: Lung Cancer and Smoking
Table 1 provides data on lung cancer and smoking status, illustrating the odds ratio calculation.
| Lung Cancer | No Lung Cancer | Total |
|---|---|---|
| Smokers (A) | 659 | (B) 984 |
| Nonsmokers (C) | 25 | (D) 348 |
| Total | 684 | 1332 |
Using the formula:
Interpretation: The odds of being a smoker are over 9 times higher in people with lung cancer compared to those without lung cancer, indicating a strong association between smoking and lung cancer.
Example: Colorectal Cancer and Family History
Another case-control example involves colorectal cancer and family history data. The odds ratio calculation is shown below:
| Colorectal Cancer | No Colorectal Cancer | Total |
|---|---|---|
| Family History (A) | 60 | (B) 120 |
| No Family History (C) | 20 | (D) 200 |
| Total | 80 | 320 |
Using the formula:
Interpretation: Individuals with colorectal cancer are 5 times more likely to have a family history of colorectal cancer compared to those without the disease, suggesting family history as a potential risk factor.
Comparison of Observational Studies
Table 2 summarizes the characteristics and differences between cross-sectional, case-control, and cohort studies.
| Characteristic | Cross-Sectional Studies | Case-Control Studies | Cohort Studies |
|---|---|---|---|
| Time | One time point | Retrospective | Prospective |
| Incidence | No | No | Yes |
| Prevalence | Yes | No | No |
| Causality | No | Yes | Yes |
| Role of Disease | Prevalence of disease | Begin with disease | End with disease |
| Assesses | Association of Risk factor and Disease | Multiple risk factors for a single disease | Single risk factor affecting multiple diseases |
| Data Analysis | Chi-square to assess association | Odds ratio to estimate risk | Relative risk to estimate risk |
Points to Remember
- Odds Ratio (OR) estimates the association strength between a risk factor and disease but does not directly imply causation.
- Case-control studies are best suited for studying rare diseases or diseases with a long latency period.
- Retrospective Design in case-control studies is subject to recall bias but is efficient in terms of time and cost compared to cohort studies.
References
- Rothman KJ, Greenland S, Lash TL. Modern Epidemiology. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2008.
- Porta M. A Dictionary of Epidemiology. 6th ed. New York, NY: Oxford University Press; 2014.
- CDC. Case-control study: A brief overview. CDC Epidemiology Guide.