address “Why” and “How” questions. For example, Manning and colleagues (1991) assess the economic costs of poor health behaviors, including a sedentary lifestyle. This information can be helpful in addressing why something should be done to increase the level of physical activity in a community. Using data from the National Health Interview Survey and the RAND Health Insurance Experiment, the authors employ an incidence-based approach to determine the lifetime costs of a sedentary lifestyle. Table 5-7 provides examples of the uses of this type of evidence.

Evidence synthesis methods encompass systematic reviews, and meta-analyses of experimental and/or quasi-experimental studies, syntheses of qualitative research, and mixed-method evidence syntheses. Evidence synthesis is particularly useful in answering “What” questions, but may also be useful for assembling information to address “Why” and “How” questions. Table 5-8 provides examples of how this type of evidence might be used.

The literature on evidence synthesis distinguishes between primary studies, or individual studies presenting original data, and systematic reviews, or organized summaries of a body of research that addresses a focused question using methods intended to reduce the likelihood of bias. Traditional methods of evidence synthesis (in the evidence-based medicine tradition) have relied heavily on quantitative methods, such as meta-analysis, to generate summaries of research-based evidence (see Hunt, 1997; Smith and Glass, 1977). Several systematic reviews relevant to obesity prevention are listed in Appendix C. Some of the limitations of this type of evidence synthesis when applied to public health issues are discussed in Chapter 3.