I am working on a meta-analysis to assess the effect of a given speech therapy.
Most papers (primary studies) use endpoints/outcomes that are standard for the intervention under scrutiny (speech rate etc), but some of them use obscure variations of those measures. I know that summary measures such as the SMD (standardized mean difference) make different scales comparable, but what if the different scales actually measure (tap into) different things, such that it would not make sense to aggregate those together even though the SMD formula allows you to?
That is to say, do I need to do a separate meta-analysis for every single measure found across all primary studies, or shall I just group several 'obscure' outcomes under a single category and explain this was done to ease the analysis, but that the measured ability might be slightly heterogeneous?
Furthermore, some studies use regional variants of that therapy that differ by a couple of (more or less important) features. How is this type of heterogeneity to be handled: should I for instance include "therapy type" as a regressor in the main analysis?
(EDIT: I believe I've addressed this second question better in a separate thread)