  Meta-analyses are commonly performed based on random-effects models, while in
certain cases one might also argue in favour of a common-effect model. One such
case may be given by the example of two "study twins" that are performed
according to a common (or at least very similar) protocol. Here we investigate
the particular case of meta-analysis of a pair of studies, e.g. summarizing the
results of two confirmatory clinical trials in phase III of a clinical
development programme. Thereby, we focus on the question of to what extent
homogeneity or heterogeneity may be discernible, and include an empirical
investigation of published ("twin") pairs of studies. A pair of estimates from
two studies only provides very little evidence on homogeneity or heterogeneity
of effects, and ad-hoc decision criteria may often be misleading.
