INTRODUCTION

In 1976, Gene Glass introduced the term' meta-analysis' to describe the statistical analysis of a comprehensive collection of research findings from individual studies. This process, which involves integrating the findings from a group of empirical studies focused on the same research question, calculates the average and variability of overall population effects (Field & Gillett, 2010; Glass, 1976; O'Rourke, 2007).

The growth of science depends on accumulating knowledge and building on the past work of others. As scientific development quickens and the amount of information in the literature continues to explode (for example, about 500,000 new articles are added to the National Library of Medicine's PubMed database each year), scientists need help to keep up with the latest research and recommended practices (Fig. 1). 

In the past, professionals depended on experts to summarize the literature and provide recommendations. However, over time, researchers started to examine the accuracy of these review articles and discovered that the evidence often did not support the recommendations. They began to promote a more scientific approach to reviews that did not rely on the subjective opinion of a single expert. This new approach required documented evidence to support claims and a systematic process conducted by a diverse team to ensure a comprehensive review of all evidence. This process is now referred to as a systematic review.