Difference-in-Differences · Policy Evaluation
Two-Way Fixed Effects DID
Use multiple units and time periods while accounting for unit-specific levels and common time shocks.
Page 1 of 7 · Big Picture
The big picture: Two-Way Fixed Effects DID
How do multiple treated and comparison units extend 2×2 DID?
Why a policy analyst should care
With many units and periods, fixed effects can summarize DID comparisons compactly, but the coefficient’s meaning depends on treatment timing and effect stability.
Follow the idea from the opening question to a practical decision. These are the conceptual moves this module will develop:
1 Question How do multiple treated and comparison units extend 2×2 DID?
2 Explore State-by-month policy data
3 Formalize Two-way fixed effects
4 Transfer Specify a multi-unit DID
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