Practice question · Multiple choice
A quasi-experiment compares crime rates in a city before and after a new law, which the researcher had no power to assign. Why does the missing random assignment weaken the causal claim, when the law itself was a genuine change imposed on everyone?
Hints
- List what else might have differed between the year before the law and the year after.
- What exactly does random assignment accomplish that this design cannot?
Show the answer
C. Because the periods may differ in ways beyond the policy.
Why
Randomisation makes groups equivalent on average across everything, including what nobody measured, and a before-and-after design differs in every respect that changes over time. The finding is ambiguous rather than merely uncertain. It is not worthless: some questions cannot be randomised, and the remedy is ruling out rivals explicitly with comparison cities and pre-trends.
Practise Quasi-Experimental and Non-Experimental Designs
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