For the risk ratio 
_{i} =
 ( a + b )( a + c )λ_{MH}


( c + d ) + [ ( a + b )λ_{MH} ]


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 
_{i}
 =
 P_{i}±
 √  
P_{i}^{2}  4 ω_{MH} (ω_{MH}  1) (a + b) (a + c)


2 (ω_{MH}  1)

where
P_{i} = (ω_{MH}  1) [(a_{i}+b_{i}) + (a_{i}+c_{i})] + n_{i}.

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 MantelHaenszel 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.