Timing and Duration of Exposure in Evaluations of Social Programs

Impact evaluations aim to measure the outcomes that can be attributedto a specific policy or intervention. While there have beenexcellent reviews of the different methods for estimating impact,insufficient attention has been paid to questions related totiming: How long after a program has begun should it be evaluated?For how long should treatment groups be exposed to a programbefore they benefit from it? Are there time patterns in a program'simpact? This paper examines the evaluation issues related totiming, and discusses the sources of variation in the durationof exposure within programs and their implications for impactestimates. It reviews the evidence from careful evaluationsof programs (with a focus on developing countries) on the waysthat duration affects impacts.