Study Designs — Cohort vs Case-Control MCQ — NEET PG Practice Question | NEETPGAI
Study Designs — Cohort vs Case-Control
medium
users PSM
Which of the following is a KEY DISADVANTAGE of case-control studies compared to cohort studies in epidemiological research?
A. Cannot establish temporal relationship between exposure and disease
B. Cannot directly calculate incidence rates
C. Prone to selection bias but not recall bias
D. Requires a large sample size and long follow-up period
Explanation
Key Disadvantage of Case-Control Studies
Key Point
The most fundamental limitation of case-control studies is that they cannot directly calculate incidence rates. Because the investigator artificially fixes the ratio of cases to controls, the sample does not reflect the true population distribution of disease, making incidence calculation impossible.
Why Option C is the MOST Fundamental Disadvantage
In a case-control study:
You select cases (people with disease) and controls (people without disease) at a fixed ratio chosen by the investigator (e.g., 1:1, 1:2, 1:4)
You then look backward to ascertain exposure history
The proportion of cases in the sample is artificially determined, not representative of the true population prevalence or incidence
Therefore, you cannot calculate true incidence in exposed and unexposed groups — the denominator (population at risk over time) is simply not available
This is why case-control studies use the Odds Ratio (OR) rather than Relative Risk (RR). The OR approximates RR only when the disease is rare in the population (< 10%) — the "rare disease assumption" (Park's Textbook of Preventive and Social Medicine).
Why Option A is Incorrect as the KEY Disadvantage
Option A states case-control studies "cannot establish temporal relationship between exposure and disease." This is partially misleading and not the most fundamental limitation:
Case-control studies can establish temporality if exposure data are collected carefully (e.g., using medical records, biological specimens stored before disease onset)
The inability to establish temporality is a practical challenge, not an inherent design impossibility
In contrast, the inability to calculate incidence is an absolute, inherent design constraint — no amount of careful data collection can overcome it
Cohort studies are superior for temporality, but this is a relative, not absolute, disadvantage of case-control design
Why Options B and D are Incorrect
Option B ("Requires large sample size and long follow-up") is actually a disadvantage of cohort studies, not case-control studies. Case-control studies are typically smaller, faster, and cheaper — this is one of their advantages.
Option D ("Prone to selection bias but not recall bias") is factually wrong. Case-control studies are classically prone to both selection bias AND recall bias (differential recall of past exposures by cases vs. controls). Recall bias is one of the most cited weaknesses of case-control design.
Comparison Table
Table
Aspect
Cohort Study
Case-Control Study
Starts with
Exposure status
Disease status
Direction
Forward (prospective)
Backward (retrospective)
Incidence calculable?
✓ Yes
✗ No (absolute limitation)
Measure of association
Relative Risk (RR)
Odds Ratio (OR)
Sample size
Usually larger
Usually smaller
Cost & time
High
Low
Recall bias
Less prone
More prone
High-Yield (Park / Mahajan): The inability to calculate incidence is the single most fundamental design limitation of case-control studies, distinguishing them structurally from cohort studies. This is why they are called "retrospective" and why they rely on OR as a proxy for RR.
Clinical Pearl
When a question asks for the key or most fundamental disadvantage of case-control studies, always choose inability to calculate incidence rates over temporality issues, because the latter can sometimes be addressed by study design modifications, while the former cannot.
Practice similar questions
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.
