[R-meta] meta-analysis of response ratios with low sample sizes

Dear Wolfgang (and users of the meta-analysis mailing list),

I am currently conducting a meta-analisis where I want to assess body size
shifts in vertebrates living in islands compared to mainland populations
(a.k.a the island rule). I am using response ratios between the mean size
of the island population and the mean size of the mainland population. In
some cases I have measurements for only 2 specimens, and I calculate mean
and SD for those 2 specimens in order to calculate the sampling variance.
However, many people would argue that calculating the SD of 2 data points
is a bit meaningless in most contexts, but in a meta-analytical context I
would expect that response ratios based on N = 2 for either the treatment
or control, or both, would be downweighted in the metaanalysis and thus it
is both informative and interesting to include them in the analyses. I
would like to know if other people have encountered these situations and
how they dealt with it. Also, what?s your opinion, Wolfgang?

I have a second query, in this same analysis I have cases where only one
specimen is measured, and thus the SD is zero. To be able to calculate the
sampling variance I add a small constant (0.5) to both the numerator and
denominator of the formula. Is this a sensible way to proceed or shall I
just discard cases where only 1 specimen is measured in either of the two
populations (or both of them)?

Thanks a lot for your time, I am looking forward to your thoughts on these
two queries.

Best,

Ana