Four principles of taking statistical uncertainty into account when estimating and reporting effects are discussed. Means of implementing the principles, ways in which the principles are violated in practice, implications for the use of multiple methods, effect size, estimation techniques, and random and nonrandom certainty are described. (TJH)

Two chapters of this issue consider critical multiplism as a research strategy with links to meta analysis and generalizability theory. The unifying perspective it can provide for quantitative and qualitative evaluation is discussed. The third chapter explores meta analysis as a way to improve causal inferences in nonexperimental data. (SLD)

The eight articles of this issue examine the nature of differences that arise between qualitative and quantitative researchers in program evaluation in terms of goals and epistemologies. The origins of these differences and their consequences are explored. Authors represent both perspectives but do not defend their ideological turfs. (SLD)

The 9 articles of this theme issue stem from a project on alcohol and drug abuse that involved 14 projects, 10 of which began with randomized clinical trials. These papers describe implementation problems associated with experimentation in field research and the focus on ensuring internal validity. (SLD)