I have little patience with scientists who take a board of wood, look for its thinnest part and drill a great number of holes where drilling is easy.

In less than a quarter-century, efforts to combine or amalgamate the results of diverse research projects have become ubiquitous. Meta-analysis, for instance, has found employment in psychology and psychotherapy, social science, education, meteorology, marketing, management and business studies, and, perhaps especially, in medicine and epidemiology, including genetics and all aspects of clinical trials, drug efficacy, disease prevention, investigator education and reporting, public health, health policy, and patient education and behavior. This chapter addresses the notion of “meta-evidence” by looking at its relation to causation and rational clinical belief. Research synthesis and systematic science raise vitally important methodological and conceptual questions and so the chapter will also identify key methodological controversies and criticisms provoked by meta-analysis and other systematic reviews. What kind of scientific evidence is a meta-analysis or a systematic review, and how best should these tools be incorporated in research and clinical practice?