I don't quite understand this. But one can often assess which effect would haveDo some kind of power analysis for each one of them or set a threshold at a certain number of participants. Include all which should have been able to reach statistical significance and assign to each one of them an effect size of 0. Throw the rest out.

been needed to reach significance, given the sample size. Maybe some value(s)

between zero and these threshold values (mean value? a random value betwee

these limits?) can be used.

It's a bit surprising that a relevant proportion of studies doesn't report descriptive

statistics together with the test results. Maybe the pages of the Cochrane colaboration

have information on how to deal with only-p-value-given results.

With kind regards

K.