Research synthesis: the state of the art.

The state of the art in research synthesis has advanced significantly in the last decade. Much of the improvement in research reviews is attributable to the application of rigorous scientific standards to the process of research synthesis. Three principal types of research syntheses are the narrative review, the replicated secondary analysis, and the meta-analysis or quantitative research synthesis. Each methodology has its strengths and weaknesses, but all have benefited from increased attention to procedural rigor. Modern methods for quantitative research synthesis have significant advantages in that they provide specific tests for the existence and consistency of relationships across studies.