The challenges of contamination in evaluations of childhood interventions

Abstract

Contamination in social programmes occurs when the control group either actively or passively receive some or all of the intervention intended for the treatment group. While contamination is considered a potential threat to the internal validity of an evaluation, there is relatively little evidence on the magnitude and prevalence of contamination in social programmes and the extent to which it can bias the estimates of programme effectiveness. The aim of this article is to document the challenges of contamination in a number of childhood interventions currently being evaluated in Ireland using experimental and quasi-experimental methods. The article documents the experiences of addressing contamination at the professional staff level, the parent and child level and the dissemination level. It describes the methods employed to minimize contamination and the procedures used for measuring it. Finally, the consequences of contamination for both programme implementation and the research design are discussed.