Improving meta-analysis for policy purposes.

Many empirical sciences have developed formal methods of combining information across independent research studies, an enterprise with a long history that was named "meta-analysis" (Hedges 1992; National Research Council 1992). When the question to be addressed is a narrow scientific one, the standard methods of metaanalysis provide adequate tools for combining the evidence. These are discussed in Cooper (1984), Hedges and Olkin (1985), Hunter and Schmidt (1990), Light and Pillemer (1984), Rosenthal (1984), or the new "Handbook of Research Synthesis" (Cooper and Hedges 1994) which includes contributions by all the authors previously mentioned.