Leading social research methodologists and evaluators address the issues of validity, research design and social experimentation in this first of two volumes inspired by the work of Donald Campbell and sponsored by the American Evaluation Association. The first part of the book focuses on three types of validity: external, construct and statistical. Much attention is given to meta-analysis, statistical testing and effect sizes. The second part is a major critique of Campbell’s vision and practice of social experimentation. Contributors review and extend Campbell’s ideas, concentrating on their relevance to evaluation and social intervention, and seeking to define an ‘experimenting society.