Case Control Studies

Question 1

Define case control study and its key characteristics:

Answer 1

A case-control study is an observational and retrospective study design that compares individuals with a disease (cases) to those without it (controls) to identify potential risk factors

• retrospective (looks at past exposures)
• efficient for rare diseases
• can’t directly measure incidence or prevalence

Question 2

Define the strengths and weaknesses of case control studies:

Answer 2

Strengths

• rare diseases and diseases with long latency can be studied
• many exposures/risk factors can be examined simultaneously
• smaller numbers required compared to cohort studies,
• relatively inexpensive, easy and quick as no waiting time or follow up required

Limitations:

• can’t establish causality as the temporal relationship is unclear
• only associations are identified
• direct incidence or relative risk estimation is not possible
• prone to recall bias as cases may remember exposures differently than controls
• multiple outcomes can’t be studied
• selection bias, poor selection of controls may distort results

Question 3

Discuss the effect estimate generated by case-control studies – Odds Ratios

Answer 3

In case-control studies, risk cannot be directly calculated because the study design does not follow individuals over time. Instead, an Odds Ratio (OR) is used to estimate the strength of association between exposure and disease

OR = Odds of exposure in cases/ odds of exposure in controls

Interpretation:

• OR = 1 → No association between exposure and disease.
• OR > 1 → Exposure is associated with higher disease risk (e.g., OR = 3 means exposed individuals are 3 times more likely to have the disease)
• OR < 1 → Exposure is protective against the disease

Question 4

Describe the Common Biases in Case-Control Studies:

Answer 4

Selection bias:
- occurs when cases and controls are not representative of the same population
- e.g, hospital-based controls may have different exposures than the general population

Recall bias:

• occurs because cases may remember past exposures differently than controls
• e.g, cancer patients may over-report past chemical exposure compared to healthy controls

Misclassification bias:

Exposure misclassification - When exposure is incorrectly measured or reported

Outcome misclassification - When some controls actually have undiagnosed disease

Question 5

Describe Case/Control Definition and Selection

Answer 5

Importance of case definition:

• cases must be clearly defined based on clinical, pathological, or diagnostic criteria
• avoid misclassification, if cases include misdiagnosed individuals, results become unreliable

Selection of controls - Controls should be representative of the population from which cases arise but must be disease-free

Question 6

What are the different sources of controls ?

Answer 6

Population-based controls – Randomly selected from the general population.
Hospital controls – Selected from hospital patients without the disease (may introduce bias).
Matched controls – Matched to cases based on age, sex, or other factors to reduce confounding.

Question 7

Describe the methods of exposure assessment and how to improve exposure assessment:

Answer 7

Methods:

Interviews/questionnaires – Self-reported data from participants (prone to recall bias).
Medical records – More objective but may have missing data.
Biological samples – Used for genetic or biomarker analysis.
Environmental exposure data – Geographic information (e.g., pollution levels).

Improvements:

• use blinded interviewers to reduce bias
• validate self-reported data with medical records
• use objective biomarkers when possible