Advice on what statistical analysis to use on my data.

Hi there,

I am looking for advice on what statistical analysis to use for a critical literature review I am working on.

Short summary: can performing yoga can decrease depression? Depressed volunteers perform yoga for various amounts of time. Detailed questionnaires are given pre and post yoga intervention to measure depression levels. The questionnaires give a value that corresponds to a level of depression. The researchers then compare these values from pre and post interventions and can determine if there was a quantitative change in depression and therefore have P values etc. These are usually compared to a control- depressed people who did not perform yoga. Sometimes pre and post yoga intervention are compared to each other and there is no separate control.

I have ~20 papers that I have chosen for this review.
My problem is that I do not know how to take these results and summarize them using my 20 research papers as my population.
I can look at the results from these papers and know that it is very positive (depression is lower after yoga). What can I do to statistically analyze and show this trend? Someone recommended doing a chi-square. I am unfamiliar with this test and mainly just looking for another opinion.

Any recommendations or comments would be greatly appreciated.
Thank you in advance.