[Process evaluation in relation to effectiveness assessment: experiences with school-based programs].

Evaluation of public health interventions usually focus on the quality of design and research methods, and less on the quality of the intervention or process evaluation. In process evaluation of school-based interventions, key issues are how completely the intervention is carried out and adherence to the protocol. In addition, exploration of intermediate variables, such as those that influence (and often predict) preventable behavior, is highly useful. This article describes the basic concepts in this topic, using examples of the effectiveness of some preventive interventions carried out in schools. The interventions discussed were mainly quasi-experimental studies, based on data from programs promoted by public health teams in the city of Barcelona. Data from process evaluation of preventive programs in secondary schools that underwent formal assessment of their effectiveness is provided. The examples are drawn from preventive programs of HIV infection or unprotected sexual intercourse (PRESSEC program) and drug consumption prevention (the PASE, PASE.bcn and x kpts programs). These examples show why the intervention process influences the impact of the programs and their results. Thorough planning of process evaluation is essential to obtain valid indicators that will identify, in the effectiveness evaluation of the intervention, the most efficacious strategies to obtain positive outcomes.