The hidden assumption that makes diff-in-diff work
A colleague estimates the effect of a new policy with difference-in-differences: they compare the before-to-after change in a treated group against the change in a control group. The arithmetic is easy, but you want to stress-test the logic.
State the key assumption difference-in-differences relies on, explain in plain terms why the method fails without it, and describe how you would check it.
Your answer
This one is open-ended. Work it through, then check your reasoning against the full solution.