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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


  • 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)
    • 4 controls per case
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

  • 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: 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.

Topic revision: r1 - 28 May 2009 - 15:07:51 - MaryB?

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