[Evaluation criteria for bronchial carcinoma. How to read a meta-analysis?]

A meta-analysis is a research procedure aiming, within the context of evidence based medicine, to combine the results of several studies in a systematic and quantifiable way. The technique is particularly useful where the individual studies give contradictory or inconclusive results. Interest in meta-analyses has increased since the early 1990's and has produced a series of publications aimed at defining the methodology and ensuring reproducible and unbiased results. The purpose of the present paper is to review briefly this methodology, to allow the critical evaluation of a meta-analysis and to discuss its internal and external validity. Meta-analysis is a powerful tool for developing recommendations for clinical practice. However, it does have limitations that must be recognised and cannot give satisfactory answers unless the individual studies are both homogeneous and of sound methodology.