Whenever a treatment is given based on a strict income threshold you have a nice natural experiment. Those who have income <= T receive treatment. Those who are

Whenever a treatment is given based on a strict income threshold you have a nice natural experiment. Those who have income <= T receive treatment. Those who are > T (even one dollar above) do not. Therefore, those who are "near" T are a great cohort to study effects of the treatment, since their socioeconomic factors are so close. It sounds like the parent comment was saying exactly this occurred for children exposed to lead, and those in the treatment group went on to have better outcomes. EDIT: Looking through the abstract, it appears the threshold was on blood / lead concentrations. This means those who differed only a tiny amount in lead concentrations received different treatments (some vs none), and had different outcomes despite being similar in all other metrics. Figure 4 was pretty interesting.