[R-meta] SMD Metric

[Yuhang Hu]

Hi Everyone,

I had a question about the SMD effect size. I read on James' website that:

"The ideal case for using the SMD metric is when the outcomes in different
studies are linearly equatable. However, if outcomes exhibit mean-variance
relationships, linearly equatability seems rather implausible."

I was wondering what is meant by linear equatability in the outcomes in
different studies and why is that needed for the use of SMD?  How could the
outcomes in different studies be perhaps non-linearly equatable or not
equatable at all (neither linearly nor non-linearly)?

(I also appreciate reference(s) that discuss such a requirement for the use
of the SMD metric)

Thank you very much for your assistance,
Yuhang

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