Textbook examples are there to illustrate a method. The recommended approach is to first give two examples where the method works, then one where it fails. But we, the textbook writers, are busy, so we’ll often just give the one that works and stop. As I wrote once, “the book has a chapter on regression discontinuity designs. That’s fine. But all we see in the chapter are successes.” — The examples you see in the textbooks are not representative of the sorts of problems you see in the real world