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Data Collectionhard
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A state implemented mandatory seatbelt enforcement on January 1, 2024. Highway fatality data from a sample of counties:

| | Before 2024 | After 2024 | |---|---|---| | Enforcement counties | 120 deaths | 105 deaths | | Non-enforcement counties | 95 deaths | 88 deaths |

A simple comparison in enforcement counties is (105−120)=−15(105 - 120) = -15(105−120)=−15 deaths. However, a difference-in-differences (DiD) analysis calculates:

DiD Effect=[Afterenforce−Beforeenforce]−[Afterno-enforce−Beforeno-enforce]\text{DiD Effect} = [\text{After}_{\text{enforce}} - \text{Before}_{\text{enforce}}] - [\text{After}_{\text{no-enforce}} - \text{Before}_{\text{no-enforce}}]DiD Effect=[Afterenforce​−Beforeenforce​]−[Afterno-enforce​−Beforeno-enforce​]

What is the DiD estimate of the policy effect?