Introduction to Case Control Study

Overview

This lecture explains the definition of case control study, its characteristics, steps, analysis, advantages-disadvantages, and important points related to exams.

Definition and Names of Case Control Study

• Case control study is also called retrospective study, trohoc study, backward looking study, effect to cause study, outcome to exposure study, risk factor study.
• The direction of the study is opposite to time, i.e., from effect (disease) to cause.
• Case: those who have the disease (e.g., lung cancer); Control: those who do not have the disease.
• Exposure history (e.g., smoking) is taken retrospectively from both groups (case/control).

Selection of Case and Control

• Case: define who will be considered a case (by diagnostic/eligibility criteria), source: hospital or general population.
• Control: similar to case in all respects, just should not have the disease; source: same hospital, relatives, neighbors, or population.
• Ratio 1:4 in small studies, 1:1 in large studies.
• In genetic studies, siblings or close relatives should not be taken as controls.

Matching and Exposure Measurement

• Cases and controls are made comparable by group or pair matching (based on age, profession, social class, etc.).
• History, questionnaire, or old records are used to know exposure.

2x2 Table and Analysis

• Patients are divided into four parts: disease+exposure (A), disease+no exposure (B), no disease+exposure (C), no disease+no exposure (D).
• In analysis, a 2x2 table is made and odds ratio = (A×D)/(B×C) is calculated.
• Incident or relative risk is not calculated in case control study, only risk estimation by odds ratio.

Interpretation of Odds Ratio

• OR > 1: Positive association (more disease in exposed)
• OR < 1: Negative association (less disease due to exposure)
• OR = 1: No association

Advantages and Disadvantages

• Advantages: cheap, fast, suitable for rare disease and long latent period, can look at multiple exposures, fewer ethical issues.
• Disadvantages: bias (selection, recall), cannot measure incidence, cannot correctly calculate relative risk.

Key Terms & Definitions

• Case — person who has the disease under study.
• Control — similar to case in all respects but without disease.
• Exposure — cause or risk factor of disease (e.g., smoking).
• Outcome — disease or effect (e.g., lung cancer).
• 2x2 Table — four-cell table for study analysis.
• Odds Ratio — relationship calculated by cross-product; (A×D)/(B×C).
• Matching — equalizing case-control groups by age, profession, etc.
• Bias — error in selection or recall affecting results.

Action Items / Next Steps

• Practice making 2x2 tables for case control study and calculating odds ratio.
• Review underlined facts asked in MCQs.
• Focus on bias and cohort study in the next class.