• The accumulation and possible combination of evidence from different studies, as contrasted with an emphasis on obtaining individually secure studies, is crucial to producing convincing evidence of generality of application and interpretation.

• Methods for combining estimates across studies include those that can be applied when the full data from individual studies are available and those that can be applied when only summary statistics are available. They make allowance for heterogeneity of the effect of interest. The methods are general and not specific to case-control studies.

• Some special considerations may be required for the combining of results from case-control studies and full cohort studies or from matched and unmatched case-control studies.

Preliminaries

The emphasis throughout this book has been on analysis and design aimed to produce individually secure studies. Yet in many fields it is the accumulation of evidence of various kinds and from various studies that is crucial, sometimes to achieve appropriate precision and, often of even more importance, to produce convincing evidence of generality of application and interpretation. We now consider briefly some issues involved, although most of the discussion is not particularly specific to case-control studies. The term meta-analysis is often used in this context. The distinctive issues are, however, not those of analysis but more those of forming appropriate criteria for the inclusion of data and of assessing how meaningfully comparable different studies really are. Important issues of the interpretation of apparent conflicts of information may be best addressed by the traditional method of descriptive review.