Interaction or Effect Modification
Interaction or Effect Modification
Interaction or Effect Modification
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Faculty Development Program<br />
Clinical Epidemiology and Clinical Research<br />
A Evans<br />
<strong>Interaction</strong> <strong>or</strong> <strong>Effect</strong> <strong>Modification</strong><br />
Clinical definition:<br />
Relationship between exposure and disease is different among different subgroups.<br />
Epidemiologic definition:<br />
Change in magnitude of an effect measure acc<strong>or</strong>ding to some third variable<br />
(after exposure and disease).<br />
Example: (coh<strong>or</strong>t study)<br />
Total Population: Heart disease No heart disease<br />
Exposed 30 50<br />
Unexposed 100 500<br />
Crude RR = 2.25; Crude OR = 3.0<br />
Males: Heart disease No heart disease<br />
Exposed 10 10<br />
Unexposed 5 100<br />
RR = 10.5; OR = 20<br />
Females: Heart disease No heart disease<br />
Exposed 20 40<br />
Unexposed 95 400<br />
RR = 1.7; OR = 2.1<br />
Confounding vs. <strong>Interaction</strong> (Rothman)<br />
1. Confounding is a bias that you hope to prevent <strong>or</strong> control.<br />
2. <strong>Interaction</strong> is just a m<strong>or</strong>e detailed description of the effect itself<br />
(in all its rich diversity).<br />
3. Confounding is something to avoid.<br />
4. <strong>Interaction</strong> is something to rep<strong>or</strong>t.<br />
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<strong>Interaction</strong>s and Clinical Trials<br />
• Extreme caution! Most clinical trials are analyzed with logistic regression.<br />
• Assess f<strong>or</strong> interaction by asking yourself: Is the risk reduction different f<strong>or</strong> different<br />
clinical subgroups (eg, different baseline risk)?<br />
• Do not believe the logistic model’s assessment of interaction.<br />
• F<strong>or</strong> imp<strong>or</strong>tant clinical subgroups, if the logistic model says there is no interaction,<br />
then you have good evidence to believe the exact opposite—that is, there is variation<br />
in treatment effectiveness!<br />
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