Meta-analysis is a statistical technique that allows you to combine effect sizes or significance testings from many different studies.

Usually translate study results into a comparative z score or correlation coefficient

Concerns are:

• You may be combining studies of unequal quality.
• The studies used different methods that might not be comparable.
• Not all studies give you the relevant statistics or information that you need to combine across studies.
• Studies that had negative results may be in the trash or file drawer somewhere and never published.