In short, a systematic review is a critical evaluation to seek the answer to a focused question in the light of available research. Although systematic reviews have a certain potential, this potential is also observed to be compromised by inadequate methods and misinterpretation of results. Systematic reviews promise a transparent and repeatable method for summarizing the literature to help improve both policy decisions and the design of new studies. The focus on evidence-based practice in many professions has increased interest in understanding both the known and unknown parts of important interventions and clinical practice. Over the past 40 years, there has been a large increase in the use of systematic reviews in both medicine and the social sciences, including psychology and education. It is known as systematic review, in which scientists systematically review the results from a large number of studies and synthesize the results in order to make inferences about the typical findings and sources of variability between studies. In the late 1970s and early 1980s, following Glass’s work, among others, Rosenthal, Glass, McGaw and Smith, Hedges, Hunter, Schmidt, and Jackson, and Light and Pillemar popularized meta-analysis and further developed the statistical methods necessary for its application. Although the term meta-analysis was first used by Glass in 1976, the first meta-analysis in the sense of combining quantitative studies is attributed to Pearson, who analyzed data from five studies on the correlations between inoculation and immunity and mortality. Despite criticism from some scientists, meta-analysis is now accepted as an appropriate method of statistically summarizing the results of individual quantitative studies in the behavioral, social, and health sciences. Glass called this method “meta-analysis”. Smith and Glass published their findings in a journal and showed that psychotherapy was actually an effective practice.

Glass calculated an overall mean value for 375 psychotherapy studies by statistically standardizing the differences between treatment and control groups. By the mid-1970s, hundreds of psychotherapy studies had produced a dizzying array of positive, neutral, and negative results, and reviews of these studies failed to settle the debate.

In 1952, Hans Eysenck initiated a fierce debate in clinical psychology by publishing a study arguing that psychotherapy had no beneficial effect on patients. An example of this situation was experienced in the field of psychotherapy in the 1950s. When we look at the research on a particular subject as a whole, we may not be able to see whether the methods applied or developed are really effective. After a certain period of time, these individual studies may reveal different findings about the subject studied. Scientific research is a cumulative process in which each scientist makes unique contributions to their area of study.

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