How to conduct meta-regression with a continuous variable measured on different scales across studies? I am conducting a meta-analysis on the efficacy of a specific type of psychotherapy for children and plan to use meta-regression to identify moderators.
The predictor I am interested in is parent psychopathology, but this variable is collected using different measures across studies. For example, Study 1 uses Questionnaire 1 (e.g., Beck Depression Inventory), Study 2 uses Questionnaire 2 (e.g., Depression Anxiety Stress Scales), and both questionnaires use different scales. What information would I need to extract and use for the predictor in the meta-regression from each study? The mean alone doesn't seem appropriate given that different measures are being used from study to study.
Thank you in advance for your input!
 A: As a beginner in meta-analysis and here at CV, I write with caution.
So far I strongly doubt that mixing scales for a predictor in meta-regression models is valid. I understand that  different scales can be mixed if they are pooled as an effect size. E.g.: SMDs in BDI mixed with SMDs in DASS works, they can further be transformed to other ES.
However, a predictor variable is not necessarily a standardized effect size and different scales usually don't meet the same metric and item thresholds. I suspect this is the case in your baseline-pathology example.
Thinking of exceptions, I would argue that:

*

*it's okay to mix different scales if they  can easily be expressed in a normed metric derived from the same reference population (hypothetically: mixing t-scores of BDI with t-scores of DASS from a representative parent sample).

*Similarly, imagine the two scales had the same item anchors (say from 1="not at all" to 6="extremely"), same number of items, and arguably same item difficulties, plus evidence that the raw values behave in similarly.

*Finally, effect size measures could predict other effect size measures, e.g.: mixed SMDs from BDI/DASS predicting 1-year relapse proportions.

I'll let you know if someone convinces me of the contrary.
Cheers
