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R11 – R00 = (R10 – R00) + (R01 - R00)
or, equivalently:
(.04 – .01) = (.03 – .01) + (.02 – .01)
0.03 = 0.02 + 0.01
R11 = R10 + R01 - R00
0.04 = 0.03 + 0.02 - 0.01
We can also express the various risks in terms of the baseline risk and the "effect" of the risk factors:
R10 = R00 + RD10 (.03 = .01 + .02) (Obesity "effect")
R01 = R00 + RD01 (.02 = .01 + .01) (Reserpine "effect")
R11 = R00 + RD01 + RD10 (.04 = .01 + .02 + .01) (Both)
Note that the word "effect" is used here by convention and for convenience, rather than to suggest
causality.
Another way we might think about these various risk equations is to try to put them all into a single
equation with "switches" for which effects are "turned on". The baseline risk R00 is always present, so
we require only two "switches", one for the obesity effect and one for the reserpine effect:
Risk = R00 + Obesity
effect
Obesity
"switch"
+ Reserpine
effect
Risk = R00 + RD10 × + 0.01 ×
Risk = 0.01 + 0.02 × + 0.01 ×
Reserpine
"switch"
When a "switch" is on (=1) then the 0.02 (obesity effect) or 0.01 (reserpine effect) comes into play,
making the Risk from the model larger.
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www.sph.unc.edu/courses/EPID 168, © Victor J. Schoenbach 13. Multicausality ― analysis approaches ― 442
rev. 10/28/1999, 11/16/2000, 4/2/2001