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Using national Medicare claims, Provider Enrollment, Chain, and Ownership System (PECOS), and Internal Revenue Service (IRS) Form 990 data from 2013 to 2019, we estimated changes in 13 quality and utilization measures following affiliation. We found that affiliation was not consistently associated with meaningful improvements in preventive care, chronic disease management, or utilization. Modest gains were observed in diabetic eye exams and organization-level care continuity, whereas primary care visits and specialist visits declined. Small POs exhibited additional gains in breast cancer screening relative to large POs, while low-quality POs and rural POs experienced lower relative performance on at least one other measure. These results suggest that health system affiliation alone does not yield broad improvements in quality across diverse types of primary care POs.<\/jats:p>","DOI":"10.1177\/10775587261472554","type":"journal-article","created":{"date-parts":[[2026,9,28]],"date-time":"2026-09-28T10:54:57Z","timestamp":1790592897000},"update-policy":"https:\/\/doi.org\/10.1177\/sage-journals-update-policy","source":"Crossref","is-referenced-by-count":1,"title":["Primary Care Physician Organization Affiliations With Health Systems Between 2014 and 2019 Were Associated With Modest Improvements in Quality of Care"],"prefix":"10.1177","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-9955-5270","authenticated-orcid":false,"given":"Justin W.","family":"Timbie","sequence":"first","affiliation":[{"name":"RAND, Arlington, VA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Maria","family":"DeYoreo","sequence":"additional","affiliation":[{"name":"RAND, Santa Monica, CA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Denis","family":"Agniel","sequence":"additional","affiliation":[{"name":"RAND, Santa Monica, CA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0615-1347","authenticated-orcid":false,"given":"Shiyuan","family":"Zhang","sequence":"additional","affiliation":[{"name":"RAND, Arlington, VA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jos\u00e9 J.","family":"Escarce","sequence":"additional","affiliation":[{"name":"David Geffen School of Medicine at UCLA, USA"},{"name":"UCLA Fielding School of Public Health, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"179","published-online":{"date-parts":[[2026,9,28]]},"reference":[{"key":"e_1_3_2_2_1","unstructured":"Avalere Health. 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