Tukey Outlier Deletion in Star Ratings, Explained | Pelica Health

Short answer

Tukey outlier deletion is the method CMS uses to remove statistical outlier contract scores before clustering when it sets Medicare Star Ratings cut points. It applies the Tukey outer-fence rule to non-CAHPS measures so a few extreme scores cannot distort the thresholds. Because there are usually more low outliers than high ones, removing them generally raises and stabilizes cut points, which makes 4 stars and above harder to reach.

What Tukey outlier deletion is

Tukey outlier deletion is a statistical step in how CMS sets Medicare Star Ratings cut points. Before CMS clusters contract scores into the 1 to 5 star bands for a measure, it removes the contracts whose scores are statistical outliers. It identifies those outliers using the Tukey outer-fence rule, a standard method that flags data points sitting far below or above the bulk of the distribution.

The sequence matters. CMS removes the Tukey outliers first, then runs mean resampling and the hierarchical clustering algorithm on the remaining scores to draw the cut points. Pulling the outliers out before clustering stops a handful of extreme contracts from dragging the thresholds toward themselves.

When CMS started using Tukey

CMS applied Tukey outlier deletion to the Star Ratings cut points for the first time with the 2024 Star Ratings. It was finalized through rulemaking and layered on top of the cut point guardrails and mean resampling CMS had already adopted to make cut points more stable from year to year. Tukey applies to the non-CAHPS measures that use clustering, not to the patient experience measures collected through CAHPS, which use a separate methodology.

How Tukey changes cut points

The effect runs in one main direction. In most measures there are more low-performing outlier contracts than high-performing ones, so cutting the outliers before clustering removes more weight from the bottom of the distribution than the top. That pulls the cut points upward.

2024

First Star Ratings year CMS applied Tukey outlier deletion

Up

General direction of cut points once outliers are removed

Non-CAHPS

Measures the Tukey outer-fence rule applies to

When Tukey took effect, many cut points rose, which made it harder for contracts to reach or hold 4 stars and above. The same method also makes cut points steadier across years, because a few extreme contracts can no longer swing where the thresholds land. The two effects are linked: the cut points are both higher and more stable.

Why Tukey matters for a quality team

Tukey raised the bar without raising any single team's measure rate. A contract can post the same raw rate it posted last year and still lose a star if the cut point moved above it. That is the trap. The thresholds are not published until after the measurement year closes, so a team that targets last year's cut points is aiming at a line that has likely already moved up.

The defensible response is to forecast where cut points are likely to land and manage to that forecast during the year, rather than reconciling against fixed historical thresholds after the year is over.

Common mistakes teams make with Tukey

• Targeting last year's cut points. Tukey tends to push thresholds up, so matching last year's rate can still drop a star this year.
• Assuming stability means easier. Tukey makes cut points more predictable, but predictable does not mean lower. The stable level is often a higher one.
• Treating Tukey as a one-time event. It applies every Star Ratings year now, not just the year it was introduced.
• Forecasting too late. Cut points are set retrospectively, so the work to clear a higher bar has to happen during the measurement year, before the thresholds are known.

How Pelica handles cut point risk

Pelica's Quality and Stars Copilot runs glide-path forecasting for HEDIS and Star Ratings measures, projecting where cut points are likely to land under the current methodology so teams close gaps before the thresholds are set rather than after. On the three triple-weighted Part D adherence measures, customers hold 96 percent medication adherence.

Related terms

Tukey is one step in how cut points are set. See Cut Points for the thresholds Tukey feeds into, and Triple-Weighted for why a star lost on an outcome measure costs three times as much in the overall rating. PDC is the adherence measure most exposed to a rising cut point.

Sources

• CMS: Medicare 2025 Part C and D Star Ratings Technical Notes (cut point methodology, Tukey outlier deletion, mean resampling)
• CMS: 2024 Medicare Advantage and Part D Star Ratings Fact Sheet (first application of Tukey)

Frequently asked questions

Common questions about Tukey outlier deletion from quality and Stars teams.

What is Tukey outlier deletion in Star Ratings?

Tukey outlier deletion is a statistical method CMS uses to remove outlier contract scores before it sets Star Ratings cut points for non-CAHPS measures. It applies the Tukey outer-fence rule, removing data points that fall far below or above the bulk of contract performance, so a few extreme scores cannot distort the cut points. CMS removes the outliers first, then runs mean resampling and hierarchical clustering on the remaining scores to set the thresholds.

When did CMS start using the Tukey method?

CMS applied Tukey outlier deletion to the Star Ratings cut points for the first time with the 2024 Star Ratings. It was finalized in rulemaking and phased in alongside the existing guardrail and mean-resampling rules that were already used to stabilize cut points.

How does Tukey affect cut points?

Because there are usually more low-performing outliers than high-performing ones, removing the outliers before clustering tends to pull the cut points upward. The practical effect is that many cut points rose when Tukey took effect, which made 4 stars and above harder to reach. The method also makes cut points more stable year to year by stopping a handful of extreme contracts from moving the thresholds.

Does Tukey apply to CAHPS measures?

No. Tukey outlier deletion applies to non-CAHPS measures. The patient experience measures collected through CAHPS use a different cut point methodology, so the Tukey outer-fence rule does not apply to them. For the clinical and process measures that use clustering, CMS removes Tukey outliers before setting the thresholds.

Why does Tukey matter for a quality team?

Tukey raised the bar. Because it tends to push cut points up, a contract can hold the same raw measure rate as last year and still drop a star if the cut point moved above it. Teams that forecast where cut points are likely to land, rather than aiming at last year's thresholds, are the ones that protect their rating, since the cut points are not known until after the measurement year closes.

Forecast the cut points Tukey moves, before the year closes.

Pelica's Quality and Stars Copilot projects where cut points are likely to land under the current methodology and closes gaps during the measurement year, not after CMS publishes the thresholds. On the three triple-weighted Part D adherence measures, customers hold 96 percent medication adherence.