An historical perspective on meta-analysis: dealing quantitatively with varying study results.

It was not until the 17th century, when the French mathematician Blaise Pascal developed mathematical ways of dealing with the games of chance used for gambling, that a science for dealing quantitatively with varying observations started to emerge. Whereas in games of chance these mathematical approaches allowed one to determine the value of possible gambles, it turned out they also allowed one to determine the best way to compare and combine observations made by different astronomers. In the 1700s, there was not yet the strong and clear distinction made today between observations within a given study, and summarized results from different studies. These ideas were tackled in the 18th and 19th century by astronomers and mathematicians such as Gauss and Laplace1 and presented in a textbook published by George Biddell Airy,2 the British Astronomer Royal. But it was only in the 20th century that statisticians addressed similar questions for the combination of clinical trial results. Summarizing results from different studies eventually became the formalized technique we refer to today as meta-analysis.