Original teaching scenarios; these are not reports of actual studies. Inspect the evidence, then judge the design to reveal the reasoning. Your first judgment counts.
Design 1 of 60 / 0 correct
Credible. Complicated. Or a walking red flag?
INSTRUMENTAL VARIABLES
Distance to training centre
A study uses distance to a training centre as an instrument for attendance, to estimate the effect of training on earnings.
+ Ask for more evidence
Distance predicts attendance, but it also affects access to jobs directly. There is a plausible path from the instrument to earnings that bypasses training.
Judge the specific claim under the stated conditions. These are teaching examples, not universal verdicts on a method.