EXTERNAL VALIDITY AND EVALUATION RESEARCH: A Codification of Problems.

This paper delimits and explicates threats to external validity particularly problematic in evaluation research. Five categories of factors are discussed: selection effects, measurement effects, confounded treatment effects, situational effects, and effects due to differential mortality. The paper focuses on pointing up specific ways in which each of the factors threaten generalizability and possible solutions to the methodological problems presented. [ABSTRACT FROM AUTHOR]