I am conducting a three-level meta-analysis using metaSEM in R. I want to find out the mean effect size for four subgroups of my dataset and later I also want to use these groups as a moderator.
My question is: Why do the estimates differ when I conduct an analysis with a categorial moderator in the meta3 function vs. when I conduct an analysis with a subset of the studies? The four groups of my categorial variable outcat are mot, ent, ges and lei.
This is the code when I use outcat as moderator:
summary(Model2 <- meta3(y=d, v=vd, intercept.constraints = 0, x=cbind(mot, ent, ges, lei), cluster = sid,data=myData, model.name = "Model 2"))
This is the code for the subset (mot as an example):
submot <- subset(myData,outcat==1) summary(Model1 <- meta3(y=d, v=vd, cluster=sid, data=submot, model.name="Model 1"))
Means for the four groups are (outcat as moderator vs subset for each group)
- 0.40 vs 0.23
- 0.74 vs 0.81
- 0.16 vs 0.24
- 0.30 vs 0.18
I have found this thread Why am I getting different means when conducting multilevel meta-analysis with factorial moderator vs. as subgroups? but somehow the answers did not help me that much and also I am using metaSEM so I don't know if that might make a difference.
As the results are so different, I don't know which one I should report or if I have made a mistake and they should actually be the same. In my report, I want to include one table with the main effect for each group and one table for the moderator analysis (is one group significantly higher than the others?). I am wondering if these two tables should be the same (because I can use the estimates from the moderator analysis as main effects for the four groups) or if I should make two tables (one for a subgroup analysis and one for a moderator analysis).