Hi Rose,

I'll write the definitions in terms of probabilities of disease and exposure.

Odds Ratio = odds of disease given exposure / odds of disease given non-exposure,

where

odds of disease given exposure = P(D+|E+) / P(D-|E+) = P(D+|E+) / (1 - P(D+|E+)),

and

odds of disease given non-exposure = P(D+|E-) / P(D-|E-) = P(D+|E-) / (1 - P(D+|E-)).

Risk Ratio = risk of disease given exposure / risk of disease given non-exposure,

where

risk of disease given exposure = P(D+|E+),

and

risk of disease given non-exposure = P(D+|E-).