IE:DPC, Ch04: Exampled of Failed ATE Identification from Self-Selection (Job Training)
Channel: David Kaplan
Date: 10/12/2022

Example where individuals are randomized into treatment/control groups to study the effect of a job training program on income, but some treatment-assigned individuals fail to show up to the training, for reasons that also cause them to have lower income anyway.  This leaves only higher-income individuals in the training program, which makes the program appear effective even when it actually has zero effect.
Textbook (IE:DPC) "Introductory Econometrics: Description, Prediction, and Causality"
https://kaplandm.github.io/teach.html
https://www.themizzoustore.com/p-236916-introductory-econometrics-description-prediction-and-causality.aspx