Meta-analysis was developed as a means of evaluating and comparing results among various studies. It was developed in order to provide a more objective method of drawing conclusions when there are multiple studies with varied results (Cooper et al. 2009). The typical method prior to the development of meta-analysis was the literature review in which studies were reviewed and results compared in order to arrive at general conclusions regarding the research question. Because different studies sometimes have different and even contradictory results, drawing conclusions about the general research question can be subjective. Rather than relying on a just box score (e.g., three studies for, two studies against), the meta-analysis calculates effect sizes (which see) and combines these in an objective formula, thereby increasing the probability that different readers will come to the same conclusions (Hunter and Schmidt 2004).
Later, methods were derived to compare studies based on...