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Control Variables
- Xit: a vector of other explanatory variables for hospital i in year t
- % Medicare patient days
- % Medicaid days
- The hospital mean of the Charlson comorbidity score
- % surgical discharges
- Market concentration (HHI)
- County-level per capita income
- County-level population density
- Zt is a vector of the year dummy variables which will adjust the effects of unmeasured, time-specific factors
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