A brief history of research synthesis.

Science is supposed to be cumulative, but scientists only rarely cumulate evidence scientifically. This means that users of research evidence have to cope with a plethora of reports of individual studies with no systematic attempt made to present new results in the context of similar studies. Although the need to synthesize research evidence has been recognized for well over two centuries, explicit methods for this form of research were not developed until the 20th century. The development of methods to reduce statistical imprecision using quantitative synthesis (meta-analysis) preceded the development of methods to reduce biases, the latter only beginning to receive proper attention during the last quarter of the 20th century. In this article, the authors identify some of the trends and highlights in this history, to which researchers in the physical, natural, and social sciences have all contributed, and speculate briefly about the "future history" of research synthesis.