[The results of a single clinical trial-is it sufficient information?]

Meta-analysis is a systematic and quantified synthesis of all clinical trials studying the same question. It is systematic because it implies an exhaustive search for all trials of the studied treatment, favourable or not, published and unpublished. It is quantified because it is based on statistical calculations leading to an estimation of the degree of the effect of the treatment. The use of statistical techniques allows to take into account the fact that different results can be obtained in different studies only by chance. Meta-analysis can be used to assess the efficacy of a treatment or to study the factors that modify this efficacy.