{ "study": { "slug": "mssp-aco-participation-concentration-2026", "title": "Medicare's largest value-based program is a long tail with a heavy head", "standfirst": "The Medicare Shared Savings Program looks broad — 511 accountable care organizations for 2026, holding 15,329 participant organizations. But its weight sits in a small head: the largest 10% of ACOs hold 43.5% of all participants, the smallest half just 11.8%, and the full-risk Enhanced track carries 71.0% of the network.", "desk": "access", "article_type": "Original Research", "published": "2026-06-16", "issue": 80, "doi": "10.5072/fonteum/mssp-aco-participation-concentration-2026", "url": "https://fonteum.com/research/mssp-aco-participation-concentration-2026", "methodology_version": "cms-mssp-aco/v1" }, "data_as_of": "2026-06-16", "datasets": [ { "slug": "cms-provider-data-catalog", "name": "CMS Provider Data Catalog", "publisher": "CMS — Provider Data Catalog", "upstream_url": null } ], "key_findings": [ { "number": "43.5%", "finding": "of the 15,329 participant organizations on CMS's MSSP file sit in the largest 10% of accountable care organizations — 51 of the 511 ACOs. The largest 5% (26 ACOs) hold 29.7%, and the ten biggest ACOs alone hold 17.3%. The program is broad on paper and concentrated in practice", "dataset": "cms-provider-data-catalog" }, { "number": "71.0%", "finding": "of all participant organizations belong to ACOs in the Enhanced track — CMS's full two-sided-risk model. Enhanced ACOs are 57.9% of ACOs by count but carry 71.0% of the network because they are larger: 36.8 participants on average against 20.7 for the lower-risk Basic track", "dataset": "cms-provider-data-catalog" }, { "number": "11.8%", "finding": "is all the smaller half of the program holds: the 256 smallest ACOs share 11.8% of participant organizations. 125 ACOs (a quarter of the program) have five participants or fewer — 2.0% of the network between them — and 44 ACOs are a single participant organization", "dataset": "cms-provider-data-catalog" }, { "number": "822", "finding": "participant organizations sit in the single largest ACO, against a median ACO size of 17 and a mean of 30.0. The gap between median and mean is the signature of the long tail — most ACOs are small, a few are very large, and the average sits far above the middle", "dataset": "cms-provider-data-catalog" }, { "number": "511", "finding": "accountable care organizations make up the published PY2026 file, holding 15,329 participant organizations across 15,293 distinct legal business names, CMS release dated 2026-01-22. The public file carries no participant TIN or NPI, so every figure is a count at the ACO, track, or program level — no individual provider is named, ranked, or scored", "dataset": "cms-provider-data-catalog" } ], "faqs": [ { "q": "What is an ACO in the Medicare Shared Savings Program?", "a": "An accountable care organization (ACO) is a group of doctors, hospitals, and other providers that join together to take coordinated responsibility for the cost and quality of care for a defined population of Medicare patients. Under the Shared Savings Program, an ACO that keeps spending below a benchmark while meeting quality standards shares in the savings; in the higher-risk tracks it also shares in the losses. CMS publishes the list of legal entities participating in each ACO by performance year." }, { "q": "How concentrated is the Medicare Shared Savings Program?", "a": "Heavily, despite looking broad. The 2026 file lists 511 ACOs holding 15,329 participant organizations, but the largest 10% of ACOs (51 of them) hold 43.5% of all participant organizations, and the largest ten ACOs alone hold 17.3%. The smallest half of the program — 256 ACOs — holds just 11.8%. A quarter of all ACOs have five participants or fewer." }, { "q": "What is the difference between the Basic and Enhanced ACO tracks?", "a": "They are the program's two risk models. The Basic track is a glide path with limited or no downside risk, intended for newer or smaller ACOs; the Enhanced track is the full two-sided-risk model, where an ACO can earn a larger share of savings but is also liable for a larger share of losses. In the 2026 file, Enhanced ACOs are 57.9% of ACOs by count but hold 71.0% of all participant organizations, because they tend to be larger — 36.8 participants on average against 20.7 in Basic." }, { "q": "Does being in a large ACO mean a provider is better?", "a": "No. An ACO participant list records that a legal entity enrolled in a Shared Savings Program ACO for a given year — nothing more. It is not a measure of cost performance, shared savings achieved, clinical quality, or outcomes, and it carries no endorsement. This study counts how participation is distributed across ACOs; it draws no inference about any provider or any ACO's results." }, { "q": "Where are Medicare ACOs concentrated geographically?", "a": "The file's service-area field is a CMS label rather than a clean state key, and most ACOs span more than one state — 311 of the 511 carry a multi-state label. Among the single-state labels that recur, California, Florida, and Texas appear most often, the same large markets that anchor most Medicare provider geographies. Because the field is frequently multi-state, this study treats geography as a labelled characteristic, not a precise state ranking." }, { "q": "Can I reproduce these figures?", "a": "Yes. Every number is a direct count over the public cms_mssp_aco_participants table — CMS's MSSP Accountable Care Organizations (ACO Participants) file, performance year 2026, CMS release dated 2026-01-22 — with no modeling. The exact SQL for the size distribution, the concentration curve, the risk-track split, and the revenue-tier check is published in the reproducibility block below." } ], "citation": { "apa": "Fonteum Research. (2026, June 16). Medicare's largest value-based program is a long tail with a heavy head. Fonteum Research, Issue 80. https://doi.org/10.5072/fonteum/mssp-aco-participation-concentration-2026", "url": "https://fonteum.com/research/mssp-aco-participation-concentration-2026" }, "reproducible_sql": "-- How concentrated is Medicare's largest value-based-care program? The Medicare\n-- Shared Savings Program (MSSP) looks like a broad network — 511 accountable\n-- care organizations (ACOs) for performance year 2026 — but its weight sits in\n-- a small head. Fully reproducible query.\n--\n-- Question: across the published MSSP ACO Participants file, how are the 15,329\n-- participant organizations distributed across the 511 ACOs, which risk track\n-- and revenue tier do the large ACOs sit in, and how concentrated is the\n-- network? The lead figure: the largest 10% of ACOs (51 of 511) hold 43.5% of\n-- every participant organization on the file. MSSP participation is a\n-- value-based-care program ENROLLMENT record — it is NOT a measure of an ACO's\n-- shared-savings performance, cost, quality, or outcomes of any kind.\n--\n-- Source:\n-- public.cms_mssp_aco_participants — CMS \"Accountable Care Organizations\"\n-- (ACO Participants) public-use file, published annually via the CMS data\n-- catalog (data.cms.gov, Medicare Shared Savings Program). 15,329\n-- participant rows; 511 distinct ACOs; performance year 2026; CMS release\n-- 2026-01-22. Public, read-only. License: US-Government-Works (17 U.S.C.\n-- Sec. 105). methodology_version = 'cms-mssp-aco/v1'.\n--\n-- Universe: this study reads the published file AS A WHOLE — every row is one\n-- participant legal business name (LBN) that CMS lists as enrolled in an MSSP\n-- ACO for PY2026. The public file carries NO participant TIN or NPI (those are\n-- restricted to ResDAC), so there is no individual-provider identifier here;\n-- the unit is the participant organization (LBN), not a person. No individual\n-- provider is named, ranked, or scored. Counts are exact over the file.\n--\n-- Counting note: \"participant organization\" = one row (one LBN under one ACO).\n-- \"ACO size\" = the count of participant LBNs in an ACO. 15,293 distinct LBNs\n-- across 15,329 rows (a handful of LBNs appear under more than one ACO).\n\n-- ============================================================================\n-- (1) Universe reconciliation — the published file at a glance.\n-- ============================================================================\nSELECT\n count(*) AS participant_rows,\n count(DISTINCT aco_id) AS acos,\n count(DISTINCT participant_lbn) AS distinct_lbn,\n count(DISTINCT performance_year) AS performance_years,\n max(performance_year) AS performance_year,\n max(source_release_date) AS cms_release,\n max(methodology_version) AS methodology_version\nFROM public.cms_mssp_aco_participants;\n-- participant_rows 15,329 · acos 511 · distinct_lbn 15,293\n-- performance_years 1 · performance_year 2026 · cms_release 2026-01-22\n-- methodology_version cms-mssp-aco/v1\n\n-- ============================================================================\n-- (2) HEADLINE: ACO size distribution. Each ACO's size = its count of\n-- participant organizations. The network is a long tail with a heavy head:\n-- a quarter of ACOs (125) hold 5 participants or fewer — just 2.0% of the\n-- network — while 17 ACOs of 121+ participants hold 23.2%.\n-- ============================================================================\nWITH sz AS (\n SELECT aco_id, count(*) AS p\n FROM public.cms_mssp_aco_participants\n GROUP BY aco_id\n)\nSELECT\n CASE\n WHEN p BETWEEN 1 AND 5 THEN '1-5'\n WHEN p BETWEEN 6 AND 15 THEN '6-15'\n WHEN p BETWEEN 16 AND 30 THEN '16-30'\n WHEN p BETWEEN 31 AND 60 THEN '31-60'\n WHEN p BETWEEN 61 AND 120 THEN '61-120'\n ELSE '121+'\n END AS size_band,\n count(*) AS acos,\n sum(p) AS participants,\n round(100.0 * sum(p) / (SELECT sum(p) FROM sz), 1) AS pct_of_network\nFROM sz\nGROUP BY size_band\nORDER BY min(p);\n-- 1-5 125 acos · 314 participants · 2.0%\n-- 6-15 109 acos · 1,137 participants · 7.4%\n-- 16-30 124 acos · 2,651 participants · 17.3%\n-- 31-60 87 acos · 3,560 participants · 23.2%\n-- 61-120 49 acos · 4,113 participants · 26.8%\n-- 121+ 17 acos · 3,554 participants · 23.2%\n\n-- ============================================================================\n-- (3) Concentration, computed directly. Rank ACOs by size and read the\n-- cumulative share off the top. The largest 10% of ACOs (51) hold 43.5% of\n-- all participant organizations; the smallest half (256) hold 11.8%.\n-- Median ACO = 17 participants, mean = 30.0, largest = 822, smallest = 1.\n-- ============================================================================\nWITH sz AS (\n SELECT aco_id, count(*) AS p FROM public.cms_mssp_aco_participants GROUP BY aco_id\n), ranked AS (\n SELECT p, row_number() OVER (ORDER BY p DESC) AS rk, sum(p) OVER () AS tot\n FROM sz\n)\nSELECT\n (SELECT round(avg(p), 1) FROM sz) AS mean_size,\n (SELECT percentile_cont(0.5) WITHIN GROUP (ORDER BY p) FROM sz) AS median_size,\n (SELECT max(p) FROM sz) AS largest_aco,\n (SELECT min(p) FROM sz) AS smallest_aco,\n (SELECT count(*) FROM sz WHERE p = 1) AS single_participant_acos,\n (SELECT round(100.0 * sum(p) / max(tot), 1) FROM ranked WHERE rk <= 51) AS top10pct_share,\n (SELECT round(100.0 * sum(p) / max(tot), 1) FROM ranked WHERE rk <= 26) AS top5pct_share,\n (SELECT round(100.0 * sum(p) / max(tot), 1) FROM ranked WHERE rk <= 10) AS top10_aco_share,\n (SELECT round(100.0 * sum(p) / max(tot), 1) FROM ranked WHERE rk > 255) AS bottom50pct_share\nFROM ranked\nLIMIT 1;\n-- mean_size 30.0 · median_size 17 · largest_aco 822 · smallest_aco 1\n-- single_participant_acos 44\n-- top10pct_share 43.5% (51 ACOs) · top5pct_share 29.7% (26 ACOs)\n-- top10_aco_share 17.3% (10 ACOs) · bottom50pct_share 11.8% (256 ACOs)\n\n-- ============================================================================\n-- (4) The full-risk track carries the network's weight. CMS runs MSSP on two\n-- risk models: BASIC (a glide path, lower downside risk) and ENHANCED (the\n-- full two-sided-risk model). Enhanced ACOs are 57.9% of ACOs but hold\n-- 71.0% of all participant organizations, because they are larger on\n-- average (36.8 participants vs 20.7 for Basic). Risk and scale move\n-- together. These are mutually exclusive per ACO (296 + 215 = 511).\n-- ============================================================================\nWITH aco AS (\n SELECT aco_id, bool_or(enhanced_track) AS enhanced, count(*) AS p\n FROM public.cms_mssp_aco_participants\n GROUP BY aco_id\n)\nSELECT\n CASE WHEN enhanced THEN 'Enhanced (full risk)' ELSE 'Basic (glide path)' END AS track,\n count(*) AS acos,\n round(100.0 * count(*) / sum(count(*)) OVER (), 1) AS pct_of_acos,\n sum(p) AS participants,\n round(100.0 * sum(p) / sum(sum(p)) OVER (), 1) AS pct_of_network,\n round(avg(p), 1) AS mean_size\nFROM aco\nGROUP BY enhanced\nORDER BY participants DESC;\n-- Enhanced (full risk) 296 acos · 57.9% · 10,885 participants · 71.0% · mean 36.8\n-- Basic (glide path) 215 acos · 42.1% · 4,444 participants · 29.0% · mean 20.7\n\n-- ============================================================================\n-- (5) Revenue tier is NOT size. CMS classifies each ACO as HIGH- or\n-- LOW-revenue by the ratio of its participants' Medicare revenue to its\n-- assigned-beneficiary spending — a structural label about practice type\n-- (hospital-anchored vs physician-led), not about how many organizations an\n-- ACO contains. The two tiers' mean participant counts are nearly identical,\n-- so the concentration in (2)-(4) is not a revenue-tier artifact.\n-- ============================================================================\nWITH aco AS (\n SELECT aco_id,\n bool_or(high_revenue_aco) AS high_rev,\n bool_or(low_revenue_aco) AS low_rev,\n count(*) AS p\n FROM public.cms_mssp_aco_participants\n GROUP BY aco_id\n)\nSELECT\n count(*) FILTER (WHERE high_rev) AS high_revenue_acos,\n round(avg(p) FILTER (WHERE high_rev), 1) AS high_rev_mean_size,\n count(*) FILTER (WHERE low_rev) AS low_revenue_acos,\n round(avg(p) FILTER (WHERE low_rev), 1) AS low_rev_mean_size\nFROM aco;\n-- high_revenue_acos 186 · high_rev_mean_size 31.0\n-- low_revenue_acos 325 · low_rev_mean_size 29.4\n\n-- ============================================================================\n-- (6) Service area is a CMS label, not a clean state key. Most ACOs operate\n-- across more than one state, so aco_service_area is frequently a\n-- multi-state string. Of 281 distinct service-area labels, 242 span more\n-- than one state, and 311 of the 511 ACOs carry a multi-state label. The\n-- single-state labels that recur most are CA, FL, and TX — the same large\n-- markets that anchor most Medicare provider geographies.\n-- ============================================================================\nSELECT\n (SELECT count(DISTINCT aco_service_area) FROM public.cms_mssp_aco_participants)\n AS distinct_service_areas,\n (SELECT count(DISTINCT aco_service_area) FROM public.cms_mssp_aco_participants\n WHERE length(aco_service_area) > 2) AS multistate_labels,\n (SELECT count(DISTINCT aco_id) FROM public.cms_mssp_aco_participants\n WHERE length(aco_service_area) > 2) AS acos_multistate;\n-- distinct_service_areas 281 · multistate_labels 242 · acos_multistate 311\n-- single-state labels recurring most: CA 25 ACOs · FL 18 · TX 14 · PA 10 · NY 10", "license": "U.S. Government Works (federal sources; 17 U.S.C. §105)", "generated_by": "Fonteum — https://fonteum.com", "notes": "Aggregate, source-traced figures frozen to the snapshot above. Reproduce by running reproducible_sql against the cited federal dataset; no per-entity records are included." }