Table A3.

Clustering Standard Errors

	Clustering	Additional Notes
ACOs
Maine	Provider organization level, eg, the health home, primary care provider, or emergency department	Accounts for clustering of beneficiaries within the provider through which they were attributed to the ACO or the comparison group
Minnesota	Beneficiary level	Did not cluster at the provider level because accurately identifying organizational clusters over time would require making several ad hoc assumptions to track organization NPIs across observation periods and because organization NPIs do not represent all of the treatment providers with whom beneficiaries actually engage. Accordingly, these factors would greatly reduce our confidence that clustering at an organizational level is correcting the bias in our standard errors.
Vermont	Provider level, eg, provider participating in the ACO or participating not in the ACO and thus in the comparison group	Provider is identified as the NPI number
BHH
Maine	Provider organization level, ie, BHH	None

Abbreviations: ACO, accountable care organization; BHH, behavioral health home; NPI, National Provider Identifier.