[A conceptual framework for the evaluation of health programs].

Evaluation can have three distinct goals: to help plan and develop programs, to provide information which will improve programs and to determine the outcomes and impact of programs. Evaluation questions flow from each of these goals. Described in this paper are different types of evaluation useful in answering these questions. Whatever type of evaluation is chosen, the evaluator must attempt to assess its validity. This validity concerns the equivalence between answers to evaluation questions and the "true value" of the program. It is just as concerned with measurement strategy and instruments as it is with the designs and techniques of analysis used, and the population studied.