[Introduction to traditional meta-analysis].

Currently there is a steady growth of scientific information in medicine which unable complete knowledge of all the articles published. Meta-analysis are studies that aim to compile all available information, grouping them according to a specific outcome, and evaluating it through methodological quality tools. Its main objective is to estimate a summary effect size after combining the individual results of each study selected under a sufficiently valid statistical analysis. The following article is to describe the basic and fundamental concepts of traditional meta-analysis, taking this information as input for future reviews of "advanced methods in meta-analysis", such as indirect meta-analysis or network analysis.