[Inclusion of research quality in meta-analyses].

The objective of this review is how study quality is taken into account in the synthesis of information from several studies. Meta-analysis has been criticized for failing into account differential quality of evidence in primary studies, allowing poor studies to drive out good science by weight of numbers. Notwithstanding there is presently no agreement on the inclusion of study quality score in quantitative meta-analysis; on the other hand, agreement does exist on knowing study quality. However, and although there is a certain level of consensus about what a good study is and on what characteristics it should have, no evaluation protocol on study quality is uniformly accepted. Furthermore, there are no data on the reliability and validity of quality questionnaires. There are several procedures to include study quality in meta-analysis: a) it is generally accepted that bias-free studies should be pooled only; b) most researches reject to include quality scores in the weight; and c) no agreement is reached on the use the quality scores as a covariate in graphical methods and regression analysis. It may be superfluous or misleading.