Homogeneity calculations

Calculate Q, where:

Q = å [weight_i x (lnOR_mh - lnOR_i)²]

To interpret, use the chi-square distribution where the degrees of freedom = S - 1 (where S is the number of studies). If p < 0.05, then there is significant heterogeneity. Another way to do it is simply to compare Q with S - 1; if Q > S-1, there is significant heterogeneity (Fleiss JL, Gross AJ. Meta-analysis in epidemiology, with special reference to studies of the association between exposure to environmental tobacco smoke and lung cancer: a critique. J Clin Epidemiol 1991; 44: 127-39.)

Basically, a p value that is greater than 0.05 indicates homogeneity;  some folks use a more stringent cutoff of p > 0.10 or p > 0.20 before accepting that studies are homogenous.  Most important is simply looking at the results - are all the odds ratios greater than 1 or less than 1?  Are there a couple of clear outliers?