Campbell-Style Designs
Nonrandomized Pre–Post Designs
A comparison group helps separate policy change from events that affect everyone—but only under a credible parallel-trends argument.
Page 1 of 7 · Big Picture
The big picture: Nonrandomized Pre–Post Designs
How does a comparison group turn pre–post change into difference-in-differences?
Why a policy analyst should care
A comparison group can subtract common time changes, but the causal interpretation depends on whether it supplies a credible counterfactual trend.
Follow the idea from the opening question to a practical decision. These are the conceptual moves this module will develop:
1 Question What does a comparison group add to a pre–post design?
2 Explore Difference-in-differences makes the contrast explicit
3 Formalize Two changes, then their difference
4 Transfer Audit a policy DID
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