How physicians are organized and affiliated can impact health care spending, utilization, and care delivery, but a lack of publicly available data has hindered efforts to study physician affiliations to date. In this issue of DataPoints, the HPC uses data from the Massachusetts Registration of Provider Organizations (MA-RPO) program, a first-in-the-nation statutory reporting requirement designed to collect public, standardized information on the structures and relationships of large Massachusetts providers,  to examine physician affiliations and changes in the physician market.
The dataset includes a physician roster file that contains key information about nearly all practicing physicians in Massachusetts  and allows users to track physician affiliation changes over time.
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The HPC used data from the 2015, 2017, and 2018 MA-RPO physician roster files. The physician roster contains names, National Provider Identifiers (NPIs), license numbers, specialties, employment status, practice sites, and medical group affiliations for all physicians that are part of a provider organization’s contracting network. It also identifies whether physicians are primary care providers (PCPs), specialists, or both, and whether physicians are pediatricians (physicians either holding themselves out as pediatricians or who spend a significant amount of their clinical time seeing pediatric patients).
Overview of the Massachusetts Physician Market
There were 21,358 physicians in the 2018 MA-RPO physician roster. Of these, 5,908 (27.7%) were PCPs and 15,450 (72.3%) were specialists. Since 2015, the number of physicians in the MA-RPO roster increased 3.0% (from 20,732).  The number of specialists increased more than the number of PCPs (3.4% compared to 2.0%) between 2015 and 2018.
Of the 21,358 physicians in the 2018 MA-RPO physician roster, 3,535 (16.6%) were pediatricians, an increase of 9.0% from the 3,255 reported in 2015.
Physicians Affiliated with the Ten Largest MA Provider Organizations
The share of physicians in the 10 largest provider organizations in Massachusetts increased slightly between 2015 and 2018: 18,398 physicians (88.7%) in 2015 and 19,282 (90.3%) in 2018.  The trend is similar for PCPs and specialists. In 2015, 87.0% of PCPs and 89.4% of specialists, and in 2018, 88.5% of PCPs and 90.9% of specialists were affiliated with the 10 largest provider organizations.
Primary Care Physicians
Consistent with the differences among the 10 largest provider organizations, the proportion of physicians that are PCPs in these organizations varied, from a high of 49.4% (Atrius, a physician-led provider organization) to a low of 19.1% (Partners, an AMC-anchored provider organization) in 2018 as shown in the graphic below.
The majority of pediatricians in the MA-RPO dataset were affiliated with one of the 10 largest provider organizations (93.7% in 2018, up from 93.2% in 2015).
However, within the 10 largest provider organizations, the share of physicians that are pediatricians varied substantially (ranging from 100% of the physicians at Children’s in 2018 to 2.3% of the physicians at Lahey).
Changes in PCP, Specialist, and Total Physician Counts
Some of the 10 largest provider organizations experienced growth in the number of PCPs, specialists, and total physicians while others saw declines between 2015 and 2018, as shown below. 
Movement of Physicians Between Provider Organizations
Between 2015 and 2018, some physicians joined organizations that report to the MA-RPO program and other physicians left (18.4% of the physicians reported in 2018 were newly reported since 2015, and 15.4% of the physicians reported in 2015 were no longer reported in 2018).
In the large provider organizations, the degree to which physicians remained with the same provider organization between 2015 and 2018 varied. For the 10 largest provider organizations, for each physician in the 2015 MA-RPO physician roster, the HPC identified whether that physician was still in the same provider organization’s contracting network in 2018, whether the physician was part of a different provider organization’s contracting network (either one of the 10 largest provider organizations in Massachusetts or a smaller provider organization), or whether that physician was no longer listed in the roster in 2018 (which could indicate, for example, that the physician left the state, retired, or was now affiliated with an organization with commercial net patient service revenue below the MA-RPO reporting threshold). Note, the graphic below does not show physicians added to these provider organizations between 2015 and 2018.
The physician roster changes identified reflect both changes in individual physician employment and organizational changes (e.g., corporate or contracting affiliation changes). For instance, most of the physicians that left the Wellforce network between 2015 and 2018 were the result of Southcoast Health Network ending its contracting relationship with Wellforce.
The Massachusetts physician market is dynamic and the MA-RPO dataset is a powerful tool for tracking changes in the physician market over time. More work is needed to understand and contextualize the factors that contribute to changes in the physician affiliations and the implications of those changes on health care cost, quality and access for Massachusetts residents.
 Providers with at least $25 million in commercial net patient service revenue are required to report to the MA-RPO program annually, including information about corporate and contracting affiliates. The MA-RPO data is a free, publicly available resource. All files except for the physician roster are available on the HPC website at https://www.mass.gov/service-details/ma-rpo-data. To request copies of the physician roster files, please email HPC-RPO@mass.gov.
 Registering provider organizations are required to report data regarding all physicians that are part of that organization’s contracting network, including employed physicians as well as affiliated physicians that have at least one payer contract negotiated by the provider organization. An HPC analysis found that the 2015 MA-RPO physician roster contains approximately 92% and 105% of the physicians in similar commercial datasets.
 The increase in the number of physicians reported between 2015 and 2018 could represent an increase in the number of physicians practicing in MA, an increase in the percentage of physicians in MA affiliated with an organization required to report to the MA-RPO program (e.g., through the acquisition of a smaller practice), or both.
 This share may be an overestimate because, as discussed above, smaller Massachusetts provider organizations are not required to report to the MA-RPO program and because, when a physician is affiliated with both one of the 10 largest provider organizations and an organization that is not one of the 10 largest provider organizations, the HPC assigns that physician only to the largest provider organization.
 Data represent both employed and affiliated physicians. Growth in PCP and specialist volume may represent new contracting relationships for a specific insurance product (e.g., affiliated physician groups participating in a Medicare ACO through a new contracting entity) or growth in individual PCPs and specialists employed or affiliated with a given organization.