CHAPTER 01
What the number is
CMS rates most Medicare Advantage contracts on a half-star scale from 1.0 to 5.0. The published figure is a Part C summary rating: a weighted roll-up of dozens of individual measures. CMS sorts those measures into five domains, covering staying healthy, managing chronic conditions, member experience, member complaints and changes in performance, and customer service. It then weights each measure by what kind of measure it is, so an outcome counts for more than a process step. A single number is carrying all of that.
Some contracts carry no numeric rating at all, and there are exactly two reasons for it. A contract can be marked "Not enough data available", or "Plan too new to be measured", which means a plan that has not existed long enough to score. Either way it is unrated, not zero, and it drops out of any honest average. A plan too new to be measured is not left unpaid in the meantime, and Chapter 2 has what statute gives it.
Set a rating. The spine above is what payment cares about. The chapters below explain why.
CHAPTER 02
The ladder is not evenly spaced
Two levers, three gates, and they do not line up. The rebate share moves at 3.5 and again at 4.5 (42 CFR 422.266). The quality bonus switches on at 4.0, and nowhere else (42 CFR 422.258). Below 3.5, neither lever has moved at all, so a 2.0 contract and a 3.0 contract are paid the same way.
Half a star is therefore worth wildly different amounts depending on where you are standing. Climbing from 3.0 to 3.5 moves the rebate share fifteen points. The next half-star buys nothing on the rebate, but switches the bonus on. The one after that adds five more rebate points. Then the payment ladder stops: a 5.0-star contract is paid exactly like a 4.5-star one.
Two cases sit outside that ladder. Five stars does carry a real advantage, but it is an enrollment rule rather than a payment one: a beneficiary may switch into a 5-star plan once during the year, outside the usual windows. And a brand new plan with no rating yet is not treated as a zero, which is the answer Chapter 1 left open. It gets 3.5 bonus points, and counts as 3.5-star when the rebate is worked out.
What the dial cannot show is that the bonus's 5 points are the middle of a range, not a flat rate. They double to 10 in the 279 counties that qualify for a doubled bonus. They also shrink, because every benchmark, bonus included, is capped at what the county would have been paid under the pre-2010 formula: in the 2026 ratebook that cap erases the bonus completely in 338 counties and clips it in 752 more. Across roughly a third of the country a fourth star is worth less than that flat 5 suggests, and in some places nothing at all. Which county you are in decides which of those you get, and that is The Rate Book. This page stops at the gates.
CHAPTER 03
Per contract or per member
CMS publishes ratings on contracts. Members live inside those contracts in very different numbers. Average every contract once, and a book of a few thousand people counts the same as a book of a few hundred thousand. Weight by members, and you are asking a different question: where do the people actually sit? Those two answers always land somewhere different, and they can land on opposite sides of the 4.0 quality-bonus line.
ILLUSTRATIVE BOOK (FABRICATED)
| Contract | Stars | Members |
|---|
Highlighted rows are the ones carrying the average in the view you are looking at.
WHAT THE FILE SHOWS
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CHAPTER 04
Paid on a delay
CMS names each Star Ratings file by a year, then pays the quality bonus one year later. The 2026 Star Ratings on this page (released October 2025) gate 2027 quality bonus payments. They are not the ratings that set the 2026 bonus. That earlier cycle used the same lag: the 2025 Star Ratings, published October 2024, gated 2026 pay.
Measurement years sit earlier still, which is why the same rule gets described with two different lengths. From the file's year label to the pay year is one year: 2026 ratings, 2027 payments. From the measurement behind a rating to the money that rewards it is closer to two, which is how The Rate Book describes the same lag. Either count works as long as you say which one you mean. Both land in the same place: the rating is locked long before the bid that spends it.
SOURCES
Where every claim sits
- 42 CFR 422.258(d)(7): quality bonus percentage points at 4+ stars, doubled in qualifying counties, and 3.5 points for a qualifying new plan.
- 42 CFR 422.266(a)(2): rebate percentages by star threshold (50% / 65% / 70%), and new plans counted as 3.5-star for that purpose.
- 42 CFR 422.166(e): how measures are weighted inside the summary rating. Outcome and intermediate-outcome measures carry a weight of 3 against 1 for process measures, which is what Chapter 1 means by an outcome counting for more. Improvement measures carry the highest weight, 5. Note that patient-experience and access measures dropped from 4 to 2 beginning with these 2026 Star Ratings.
- 42 CFR 422.62(b)(15): the special election period for plans rated 5 stars. A beneficiary may use it once per contract year, running from the December 8 before that year through November 30 of it. It is an enrollment right rather than a payment one: nothing above 4.5 stars changes what a plan is paid.
- The benchmark cap is SSA 1853(n)(4) against the pre-ACA amount at 1853(k)(1). The 279 doubled-bonus, 338 fully capped, and 752 partially capped county counts are recomputed from CMS's 2026 ratebook, the same file The Rate Book runs on.
- CMS Star Ratings Data Tables, published October 2025, gating 2027 quality bonus payments. Mind the vintage. Most tables in that release are dated 8 October 2025, but the summary-rating workbook behind these aggregates is CMS's 22 July 2026 republication, issued after CMS voluntarily recalculated the 2027 bonus ratings following a successful legal challenge. That recalculation dropped several measures and, by CMS's own rule, could only raise a contract's rating and never lower it. So these figures are the ratings that actually gate 2027 payment, and they sit a little above the October originals. The five domain names in Chapter 1 are CMS's own, read from the Domain Stars table in the same release. Enrollment join from CMS Monthly Report by Contract.
- Aggregates on this page cover every Medicare Advantage contract in the file, not a sample: 518 rated and 196 unrated, holding 97% of national MA enrollment. Built by star-book/scripts/build_stars_extract.py, which strips contract names, marketing names, parent organizations, and contract IDs, and refuses to write if any of them survive. Checked by verify_stars.py, which re-derives every figure from the raw CMS files through a separate reader.