A systematic and quantitative study of studies. In meta-analysis, an “observation” is a finding from a study. Meta-analysis was applied in 1904 by Karl Pearson, who combined data from British military tests and concluded that the then-current practice of vaccination against intestinal fever was ineffective (Mann 1994). Although meta-analysis had also been used for decades in personnel psychology, Glass (1976) introduced the term. In meta-analysis, one uses documented procedures to (1) search for studies, (2) screen for relevant studies, (3) code results (a survey of the authors of the studies can be used to help ensure that their findings have been properly coded), and (4) provide a quantitative summary of the findings. The primary advantages of meta-analysis are that it helps to obtain all relevant studies and that it uses information in an objective and efficient manner. Cooper and Rosenthal (1980) found that meta-analysis was more effective than traditional (unstructured) literature reviews. Meta-analyses are useful in making generalizations, such as which forecasting method is best in a given situation. Meta-analyses are also useful when estimating relationships for an econometric model (see a priori analysis). When aggregating results across studies with small sample sizes, it may be useful to follow the procedures for assessing statistical significance described by Rosenthal (1978). Since 1980, meta-analysis has been popular in many fields.