Quantitative Methods 2017

Risk vs Odds in formulas

odds = a/b

= a/(a+b-a) = a/(a+b) / (a+b-a)/(a+b) = a/(a+b) / [ (a+b)/(a+b) – a/(a+b) ] = p / (1-p )

If p (risk) is small: odds ~ p

log (odds) = log (p / (1-p))

log (p/(1-p)) = a + b  x + …

NOTE: if I apply logistic regression to a cohort study I’m estimating the odds (or odd ratio) not the risk (or relative risk), unless p is small

06/10/2017

FMB