Increasingly, researchers are interested in questions regarding treatment-effect variation across partially or fully latent subgroups defined not by pretreatment characteristics but by postrandomization actions. One promising approach to address such questions is principal stratification. Under this framework, a researcher defines endogenous…

In 2009, the government's Department for Education commissioned a team of researchers at NatCen Social Research to evaluate the effectiveness of the youth development/teenage pregnancy prevention programme 'Teens and Toddlers'. Previous studies had positive findings but had not been very rigorous in terms of methodology and methods used. We…