I have collected papers reporting no. of observed species in two types of habitats, A and A’, and been trying some outcome measures for meta-analysis. I noticed that effect size such as Hedges’ g cannot be calculated for cases where habitat A and A’ both have only one data value (e.g., 23 species in habitat A and 17 species in habitat A’). Unfortunately there are many such studies. Should I omit them from my meta-analysis?
It seems to me that omitting particular type of studies from consideration goes against the spirit of meta-analysis. I mean, if meta-analysis is meant for summing up scattered knowledges in the literature, any collection bias should be minimized. So… does anyone know how to include one-data cases into meta-analysis?