I'm running a meta-regression and am inputting at different study characteristics as IV's. The problem is some are continuous and some are categorical. The IV's are: Age Gender (2 values, dummy coded) Study design (2 values, dummy coded) Population type (4 values, dummy coded).
I did univariate analyses first, and age and study design were significant. I also did a univariate analysis with population type, and one of them was significant (although I understand this means it is only significant from the reference category).
When I put all the significant ones into a multiple meta-regression (i.e. age, study design, populationtype1, populationtype2, populationtype3), only one of the population type dummy codes is significant.
My questions are: Can you put two (or more) categorical iv's in one analysis (does the constant apply to both categorical variables e.g. the reference study design and population type)? How do I interpret this if this is the case? What further analysis should I do to unpick the effect of population type?
I hope this makes sense