For the risk ratio -
||( a + b )( a + c )λMH
|( c + d ) + [ ( a + b )λMH ]
Estimation of i
assuming a common risk ratio is straightforward using the formula given to the right. You can now see why you need to compute the common risk ratio before
testing for interaction - it is necessary because the value of the common ratio is used to assess whether interaction is present or not. But remember - if interaction is
present, then your common risk ratio is no longer meaningful.
For the odds ratio -
|Pi2 - 4 ωMH (ωMH - 1) (a + b) (a + c)
|2 (ωMH - 1)
Pi = (ωMH - 1) [(ai+bi) + (ai+ci)] + ni.
Estimation of the i assuming a common odds ratio is a bit less straightforward. We give the appropriate formula to the right. Again we use our Mantel-Haenszel estimate, this time of the common odds ratio. Note however that we have a ± sign in the equation which means that we will get two answers to the formula. We choose the answer that gives positive values to the other expected values.