Example of Case Control Study Design
Kaiser Sigmoidoscopy Study
Lead Editor(s): Jeff Martin, MD
Case-Control Study Design Choices
Kaiser Research Division 1990
- Question: Does screening sigmoidoscopy prevent colon cancer deaths?
Design choices:
- Prospective cohort: incidence about 100 deaths per yr but only about 15% of colon cancers detectable by sigmoidoscopy—10 to 20 yrs
- Retrospective cohort: Kaiser members in 18-year period--100,000’s of records to review
- Case-control design
Sigmoidoscopy Study
The design is based on the primary study base of Kaiser membership over an 18-year period.
Colon cancer deaths 1971-1988: 1712
Subjects:
- Cases = colon cancer deaths detectable by sigmoidoscopy: 261
All colon cancer deaths were identified and then the subset of those deaths that could have been detected by sigmoidoscopy.
- Controls = alive and in Kaiser at time of matched CA death (incidence-density)
Using incidence-density sampling meant that at the date of each eligible colon cancer death, 4 Kaiser members were selected at random as controls from the membership enrolled at that time (note that any one of these controls could theoretically have later become cases).
Study Protocol
Protocol:
- Blinded review of prior 10 years of medical records
The medical records of all cases and controls were reviewed to determine if they had undergone a prior sigmoidoscopy, and, if so, whether it was for indication or just a screening test.
Predictor =screening sigmoidoscopy (not incl sigmoidoscopy for indication)
- Screening sigmoidoscopy was the primary predictor.
Results
RESULTS: 8.8% of cases vs. 24.2% had prior screening sigmoidoscopy
The results showed an approximately 3-fold increase in risk of colon cancer death among those not screened.