01 · Working principle
Decompose the average
Begin with eligible discharge counts and matched-weight coverage. Then compare the distribution of MS-DRGs and identify which groups contribute most to the weighted difference. Stratify only where definitions and sample sizes support the review.
- Confirm the discharge-date fiscal year and weight file.
- Show eligible, matched, unmatched, and excluded discharges.
- Identify high-contribution DRG shifts rather than treating CMI as a black box.
- Review coding or documentation as possible lenses, not presumed causes.
02 · Working principle
What not to conclude
A higher CMI does not automatically mean patients were clinically sicker, documentation improved, revenue increased, or performance improved. Those propositions require their own evidence. CMI is useful precisely because it is bounded: an aggregate of applicable weights, not an all-purpose hospital score.
Executive checklist
What leaders should ask next
- Did the eligible denominator or match rate move?
- Which DRGs explain most of the weighted change?
- Are weights aligned to discharge-date fiscal year?
- Which interpretation remains a hypothesis?
Interpretation boundary
Limits to keep visible
- CMI is an aggregate of applicable DRG relative weights, not a complete measure of patient severity or hospital performance.
- MS-DRG and APR-DRG should not be combined into one synthetic metric.
- Classification and payment references change; use the appropriate effective version rather than automatically using the newest file.
Authoritative references
Sources and method notes
- Centers for Medicare & Medicaid Services: Case Mix Index
CMS definition and calculation context for hospital case mix index. Accessed August 19, 2026.
- Centers for Medicare & Medicaid Services: MS-DRG Classifications and Software
Official MS-DRG classifications, software, and fiscal-year materials. Accessed August 19, 2026.
- Centers for Medicare & Medicaid Services: FY 2026 IPPS Final Rule Home Page
Table 5 includes current MS-DRG relative weights and geometric and arithmetic mean length of stay; match the table to the discharge-date fiscal year. Accessed August 19, 2026.
- Agency for Healthcare Research and Quality: APR-DRG Severity of Illness
APR-DRG severity-of-illness context and four-level subclass structure. Accessed August 19, 2026.
- Solventum: APR-DRG Classification System
Official methodology overview for the proprietary APR-DRG classification system. Accessed August 19, 2026.