1. Study design V: Case-Control Studies
Principles of design and threats to validity
Brent Hagel, PhD

2. Case-control study Outcome based selection
Cases have desired characteristic:
Disease OR other outcome
Controls don’t
Analysis compares groups for:
Frequency of risk factors
Retrospective OR prospective

3. Case-control Studies
Subjects are selected on outcome status (disease / no disease)
Look back to determine exposure status
There can be prospective data collection in a case-control study
Exposure
Disease ?
Time ?
Study Begins
From Hennekens & Buring. Epidemiology in Medicine. Little Brown and Company, 1987; P. 24

4. What is the relation between maternal smoking and birth defects?
Past
Present
Maternal smoking
Yes?
No?
Case:
Infant with a Birth Defect
primary analysis
Maternal smoking
Yes?
No?
Control:
“Normal Infant”

5. Retrospective case-control study - strengths
Useful for rare outcomes
Less expensive and labour intensive than cohort study with same outcome
Efficient for diseases with long induction/latency period before disease development
Useful for hypothesis generation

6. Retrospective case-control study - weaknesses
Cannot estimate prevalence or incidence
Can be difficult to establish temporal relationship
Only a single outcome can be studied
Susceptible to information bias
Reporting, historical sources of information, recall

7. Prospective case-control study
Strengths over retrospective design
Prospective identification of subjects and data collection (exposure)
Standardisation of outcome
Weaknesses
No estimate of prevalence/incidence
Single outcomes
Waiting for sufficient outcomes to accrue (time and resources)

8. Threats to validity Selection bias? Misclassification bias?
e.g., Control group = infants with respiratory problems?
Misclassification bias?
e.g., Differential recall for cases and controls?
Confounding?
e.g., mother’s alcohol use?

9. Design a case-control study . . .
Research Question: “Are bicycle helmets effective in preventing head injuries?”
You have 30 minutes!

10. Cheat sheet Choice of design reasons for using a case-control study
reasons against a case-control study
Who would be an ideal case?
Who would be an ideal control?
Where would you obtain cases and controls?
What would be your main exposure variable and how would you measure it?
What other variables would you consider in this study?