Medicare Enrollment File Information

The Centers for Medicare & Medicaid Services (CMS) maintains an annual Master Beneficiary Summary File (MBSF) that includes all Medicare beneficiaries. This file has multiple segments, as briefly described below. A more detailed explanation can be obtained hereExternal Web Site Policy from the Centers for Medicare and Medicaid’s (CMS’s) Chronic Conditions Data Warehouse (CCW).

  1. Base (A/B/C/D)External Web Site Policy *contains information on the person’s date of birth, date of death (if any), sex, race, state of residence, and monthly enrollment in Part A (inpatient), Part B (outpatient), Part C (Medicare Advantage/ managed care/ HMO) and Part D (prescription drug coverage).
    • NOTE: The Multi-year Crosswalk File allows the same contract-plan to be tracked across years (Part D: 2007+; Part C 2015+), because contract and plan identifiers can change on an annual basis. The relationship code and relationship description variables clarify if a contract-plan in the reference year is new, renewal, consolidation, or termination.
    • NOTE: The Plan Bridge File provides a crosswalk between the encrypted and unencrypted contract and plan identifiers; in 2015, CMS began releasing unencrypted contract and plan identifiers.
    • NOTE: The Multi-year Crosswalk File and Plan Bridge File can be used together to track plans across time (e.g., if a plan used one, encrypted identifier in 2010 but then switched to a new plan identifier in 2011, use the annual Crosswalk File 2010-2011 to find that change. Then use the Plan Bridge File to link this encrypted plan identifier to its unencrypted plan identifier and then the annual Crosswalk File again to track the same plan after 2015.
  2. Plan Characteristics FileExternal Web Site Policy contains Medicare Advantage plan and Prescription Drug Plan information separated into six subfiles: base/benefit file, premium file, cost sharing tier file, service area file, special needs plans file and multi-year crosswalk file. The information in the Plan Characteristics File can be linked to the Part D Drug Event File (using contract ID and plan ID) to assess for variation in utilization and costs by plan type. Please note, plan identifiers in the data were encrypted prior to 2015; the multi-year crosswalk file allows tracking plans across time. For more information (e.g., file layout and codebooks) please visit the Chronic Conditions WarehouseExternal Web Site Policy; documentation for file years 2007-2014 is listed under the Medicare Part D heading and file years 2015+ under the Medicare Enrollment heading.

* Please note Medicare Part A and Part B claims are not reported during enrollment in Medicare Advantage.

Given there are two cohorts of persons included in the SEER-MHOS data (persons with and without cancer), there are two subsets of MBSF data available via SEER-MHOS (MBSF-Cancer and MBSF-Non-SEER); the file documentation is the same. The MBSF-Non-SEER file can be used to to identify non-cancer controls. The persons in the MBSF-Non-SEER file are subjects that are not in the SEER cancer file but have an MHOS survey in a SEER area.

Last Updated: 07 Feb, 2024