Benefit to individuals versus the population There is a longstanding debate as to when is the best age or ages to screen children for strabismus and amblyopia.1 Our understanding of the sensitive period for visual development would suggest that ideally this is as early as possible once a precipitating amblyogenic factor is present. Assuming a high sensitivity, screening at school entry allows for almost all amblyopia that is likely to occur to be detected but at a risk that treatment for longstanding amblyopia will not be as effective as it would be had it been detected earlier. This observation leads us to ask what is the quantifiable benefit of implementing an additional screening programme before school entry?—a question elegantly addressed by Williams and colleagues in this issue of the BJO (988). Williams et al 2,3 have previously reported on a randomised controlled trial (RCT) nested within a large geographically based, population birth cohort study (ALSPAC, The Avon Longitudinal Study of Parents and Children). They found what they termed “de luxe” early intensive screening (five orthoptic examinations between 8 and 37 months) referred for treatment more …