Table 1.

Variables used for study analysis categorized by Donabedian’s model

CMS Provided Variable	Description	Donabedian’s Model
Total Assigned Beneficiaries (TAB)	Number of Medicare fee-for-service beneficiaries attributed to an ACO; count provided by CMS	Structure
Total Benchmark Expenditures (TBE)	Target total expenditures are based on an ACO’s historical performance and national trends, risk adjusted, and calculated as the per capita benchmark expenditures multiplied by the total person years; dollar amount provided in CMS dataset	Process
Quality Reporting	Whether or not an ACO met the CMS quality reporting requirement	Process
Total Assigned Beneficiary Expenditures (TABE)	Actual aggregate amount CMS spent on Total Assigned Beneficiaries (TAB); dollar amount provided in CMS dataset	Outcome
Created Study Variables
Minimum Savings Rate (MSR)	Minimum rate of expenditures less than target expenditures necessary to earn shared savings payments; calculated based on CMS-defined scale (CMS 2014c)	Process
Per Capita Benchmark Expenditures (PCB)	Target expenditures per beneficiary; Total Benchmark Expenditures (TBE) / Total Assigned Beneficiaries (TAB)	Process
Per Capita Expenditures (PCE)	Average actual expenditures per beneficiary; calculated by dividing Total Assigned Beneficiary Expenditures (TABE) by Total Assigned Beneficiaries (TAB)	Outcome
Generated Shared Savings Indicator (GSSI)	Binary indicator variable (true/false) indicating whether savings were positive or negative	Outcome
Quality Score (QS)	Quality performance score for each ACO calculated based on CMS defined methodology (CMS 2014e)	Outcome
Per Capita Savings Achieved (PCSA)	Average savings achieved per beneficiary, negative savings represents losses; calculated by subtracting target expenditures from actual expenditures (Per Capita Benchmark Expenditures (PCB) - Per Capita Expenditures (PCE))	Outcome

Source: Derived or calculated based on Medicare Shared Savings Program Accountable Care Organizations Performance Year 1 Results [20].

Notes: CMS provided detailed ACO level data, in some cases actual specific count and calculated variables were included in the public data set. In other cases specific variables are calculated by CMS and provided to individual ACOs, such as Minimum Shared Savings Rate (MSR) and Quality Score (QS), but were not included in the publically available dataset. These were calculated for our analysis using the data elements provided by CMS and applying the published methodologies. The Quality Score methodology includes weighting for individual performance on each of the 33 measures. Each of these are assigned to a domain, which also carries a weight and are combined for a final quality score. For those organizations that did not successfully submit quality measures in 2013, only partial scores could be calculated, based upon the 11 quality measures supplied by CMS [1]. The Minimum Savings Rate (MSR) is used by CMS to determine the percent change in expenditures, beyond target benchmark, that each ACO must exceed in order to share in savings or pay losses. The MSR was calculated based on CMS’s published methodology [21]. The MSR is different depending on participant Track. Track 2 (with both upside and downside risk) participants are all assigned an MSR of 2.0%. For Track 1 (upside risk only) participants the MSR is a on a sliding scale based on the number of attributed beneficiaries.