In a retrospective case-control study, the initial case group is composed of persons

In a retrospective case-control study, cases and controls are selected based on disease status. The case group is composed of individuals who have the disease (cases), while the control group consists of individuals without the disease. This design allows researchers to look back in time to assess exposure to potential risk factors.

Step-by-Step Justification:

Selection of Cases and Controls:

Cases: Individuals who already have the disease.

Controls: Individuals without the disease but similar in other aspects.

Direction of Study:

A retrospective study moves backward from the disease outcome to investigate potential causes or risk factors​.

Data Collection:

Uses past medical records, interviews, and laboratory results to determine past exposures.

Common Use:

Useful for studying rare diseases since cases have already occurred, making it cost-effective compared to cohort studies.

Why Other Options Are Incorrect:

B. without the disease: (Incorrect) This describes the control group, not the case group.

C. with the risk factor under investigation: (Incorrect) Risk factors are identified after selecting cases and controls.

D. without the risk factor under investigation: (Incorrect) The study investigates whether cases had prior exposure, not whether they lacked a risk factor.

CBIC Infection Control References:

APIC Text, Chapter on Epidemiologic Study Design​.