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Total
population
(n=1,000)
Baseline 5-year follow-up Outcomes
Exposed
(n=400)
Unexposed
(n-600)
(Assume no losses to follow-up, including deaths from other causes.)
The above data are often summarized into a 2 x 2 table:
5 - year incidence of disease
Exposure
Disease Yes No Total
Yes 80 60 140
No 320 540 860
Total 400 600 1000
Affected (n=80)
Unaffected (n=320)
Affected (n=60)
Unaffected (n=540)
Note on terminology: The four numbers (80, 60, 320, 540) in the interior of the table are referred to
as the “cells”; the row and column totals (140, 860, 400, 600) are referred to as the “marginals”. The
cells are often referred to as “a”, “b”, “c”, and “d” in zig-zag fashion beginning with the upper left
cell.
CI (crude) = 140 / 1000 = .14 (i.e., the overall 5-year cumulative incidence was 14/100)
CI1 = 80 / 400 = .20, CI0 = 60 / 600 = .10
CIR = CI1 / CI0 = .20 / .10 = 2.0 (the exposure was associated with a doubling of risk)
CID = CI1 - CI0 = .20 - .10 = .10 (see below for interpretation)
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www.epidemiolog.net, © Victor J. Schoenbach 2000 7. Relating risk factors to health - 178
rev. 10/9/2000, 5/8/2001, 2/19/2002, 4/26/2002