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For an independent subgroup contrast in meta-analysis, the difference of disjoint independent subgroup estimates has estimated variance equal to the sum of the two variances. The subgroup contrast and standard error areThe Wald test of equal subgroup mean log odds ratios therefore givesUnder the null, $Z$ is approximately standard normal and $Z^2$ has a chi-squared distribution with one degree of freedom. A quantile-2 confidence interval for the difference is $(0.29-0.8,0.29+0.8)=(-0.51,1.09)$. There is no statistically persuasive evidence of a subgroup difference. The result does not establish equal effects. Nor does a between-trial subgroup comparison, by itself, identify a causal advantage of one technique: other study characteristics may differ.

Divide studies by a specified characteristic and estimate an effect in each subgroup, then test the difference using an independent subgroup contrast in meta-analysis or an appropriate interaction model. The test of a between-group difference is distinct from separate tests against no effect within each group. Exploratory splits can be affected by multiple testing and by confounding between study characteristics.