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Epidemiology: 8) Cohort Analysis - Measures of Effect


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Number needed to treet
1/(Po-P1) = -1/RD
Utility of Risk Risk Difference
RD can have more clinical/public health meaning since is characterizes absolute risk
RR vs. RD
-RR is less dependent upon the underlying population.
-RD provides incidence information.
-No methods to do multivariate RD.
2 reasons to use OR
1) Logistic regression for fixed (defined) cohort studies.
2) Case-control estimates
OR can estimate RR if
D is infrequent in both E+ and E-
min(Po,P1)<0.1 (10%)
Major problems with risk, incidence, and odds information
They do not measure the health "impact" of a particular exposure.
Attributal fraction
Proportion of the disease occurence that would be eliminated if exposure to the risk factor were prevented
Attributable fraction: Synonyms
-Etiologic fraction
-attributable proportion
-attributable risk
-population attributable risk (percent)
Who is the Attrubutable fraction calculated for?
1) E+ group
2) Entire population
Attributable fraction for the exposed (discription)
Proportion of cases in the E+ group attributable to E.
Eg. Among NSAID users, what proportion of peptic ulcers are caused by NSAIDS?
Attributable fraction for the exposed (Eqn)
(P1-Po)/P1 (risk data)
(I1-Io)/I1 (rate data)
(RR-1)/RR (risk data)
(IRR-1)/IRR (rate data)
(OR-1)/OR (rare disease assumption)
Attributable fraction for Population (disc)
Proportion of cases in the population attributable to E.
Eg. For a given patient population, what proportion of peptic ulcers are caused by NSAIDS? (need strength of the association between E and D and prevalence of E+)
Attributable fraction for Population (eqns)
(P-Po)/P (risk)
(I-Io)/I (rate)
¶(RR-1)/[1+¶(RR-1)] (risk)
¶(IRR-1)/[1+¶(IRR-1)] (rate)
¶c(OR-1)/OR (Odds)
Attributable Fraction for population (¶ and ¶c)
¶ = N1/N (proportion exposed in cohort for risk data)
¶ = T1/T (Proportion exposed for person-time for rates)
¶c = a/(a+b) (exposure prevalence in D+ for rare D)
Attributable fraction for exposed (protective exposures) (disc)
Also called prevented fraction or effictiveness.
Often used in vaccine studies
Attributable fraction for exposed (protective exposures)(eqn)
Marginal cost-effecitiveness
(Difference in cost)/(Events prevented)
See 8-12

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