Average Effect of Treatment on the Treated

A scientist develops a new drug to help reduce addiction to pain killers. He administers this drug to many recent abusers of painkillers, and finds that these patients had substantially lower rates of painkiller abuse than would be expected, given that painkiller addicts often relapse following rehab. However, across the entire human population, the scientist expects the average treatment effect of his drug to be negligible on rates of painkiller abuse. Why might the scientist expect the average treatment effect on the treated to be larger than the average treatment effect?

Possible Answers

Because those who were treated with the drug were more likely to gain benefit from using it.
Because painkiller abusers are an unreliable sample.
Because the sample who tried the scientist's drug was small.

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Average Effect of Treatment on the Treated

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