Relatively little is known of the use of systematic review and synthesis methods of non-randomised psychiatric epidemiological studies, which play a vital role in aetiological research, planning and policy-making.

To evaluate reviews of psychiatric epidemiological studies of functional mental disorders that employed synthesis methods such as systematic review or meta-analysis, or other forms of quantitative review.

We searched the literature to identify appropriate reviews published during the period 1996 to April 2009. Selected reviews were evaluated using published review guidelines.

We found 106 reviews in total, of which 38 (36%) did not mention method of data abstraction from primary studies at all. Many failed to mention study quality, publication bias, bias and confounding. In 73 studies that performed a meta-analysis, 58 (79%) tested for heterogeneity and of these, 47 found significant heterogeneity. Studies that detected heterogeneity made some allowance for this. A major obstacle facing reviewers is the wide variation between primary studies in the use of instruments to measure outcomes and in sampling methods used.

Many deficiencies found in systematic reviews are potentially remediable, although synthesis of primary study findings in a field characterised by so many sources of heterogeneity will remain challenging.