Differences in the response heterogeneity of educational attainment and achievement to the same instrumental variable

The typical econometric application of instrumental variable (IV) estimates imposes a constant-treatment-effect model for all individuals. Using regression discontinuity design, we find while older school starters tend to have lower educational attainment, they are more likely to achieve qualifications at different educational levels for the UK. This finding joins the discussion on instruments for educational outcomes by offering some evidence that school starting age can affect treated individuals diversely. We highlight that identifying the treatment effect with methods similar to IV estimates should be concerned in the context of heterogeneous treatments; that is, the instrument in question has an effect on treatment for particular subpopulations only, and those subpopulations may not be the same for different treatments.