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General Contents of the Beneficiary Eligibility and Enrollment Files

In general, the eligibility and enrollment files contain:

  • Variables used for unique patient identification across files
  • ¬†Demographic information including birth date, death date, and race
  • Variables used to track enrollment in Medicare over time.
  • Reason for Medicare eligibility (e.g., aged, disabled or End Stage Renal Disease).
  • Type of Medicare enrollment, such as enrollment in traditional, fee-for-service Medicare versus Medicare managed care (i.e., Part C)

Note: Each file includes records for all Medicare beneficiaries enrolled at some point during the calendar year.

Evolution of the Files
File Name Year Introduced Year Discontinued Created for Calendar Years Key Variables Available Notes
Denominator Prior to 1999 March 2010 1999-2009 Monthly enrollment/entitlement information for Medicare Part A, B Created in March of the following CY
State/county codes Age calculated as of the beginning of the CY
Zip code State and zip code as of March of the following CY
Date of birth Deaths reported in the file through March of the following CY
Date of death
Demographics (e.g. age, race, sex)
Beneficiary Annual Summary File (BASF) 2006 2011 1997-2010 Monthly enrollment/entitlement information for Medicare Part A, B Created at the end of the CY
State/county codes Stand-alone file containing a combination of enrollment, eligibility, and summary level variables
Zip code Included subset of Denominator variables
Date of birth Summary reimbursement variables for each claim type (IP, OP, SNF, HHA, Hospice, Carrier, DME)
Date of death Condition Categories
Demographics (e.g. age, race, sex) Summary utilization variables
Number of inpatient stays
Number of physician visits
Chronic condition flags
Beneficiary Summary File (BSF) 2007 March 2012 1999-2011 Monthly enrollment/entitlement information for Medicare Part A, B, C, and D Created at the end of the CY
State/county codes Included all Denominator variables
Zip code Included Part D Denominator variables starting in 2008
Date of birth Age calculated as of the end of the CY
Date of death State and zip code as of the end of the CY
Included new race variable
Denominator with Part D 2008 March 2010 2006-2009 Monthly enrollment/entitlement information for Medicare Part A, B, C, and D Included all Denominator variables
State/county codes Included the Part D entitlement indicators
Zip code Included new dual-eligible variable
Date of birth Included expanded race variable
Date of death
Demographics (e.g. age, race, sex)
Master Beneficiary Summary File (MBSF): Base 2012 NA 1999-current Monthly enrollment/entitlement information for Medicare Part A, B, C, and D Base file contains similar variables as those found in the BSF and Denominator with Part D
State/county codes New Medicare Advantage/Part C variables added as of 2017
Zip code Age calculated as of the end of the CY
Date of birth State and zip code as of the end of the CY
Date of death
Demographics (e.g. age, race, sex)
Medicare Advantage contract number/benefit package
MBSF: Cost and Utilization 2012 NA 1999-current Number of emergency room visits Annual, beneficiary-level summarized information on service use and Medicare payment by file type (e.g., inpatient)
Number of inpatient stays
Number of physician visits
MBSF: Chronic Conditions 2012 NA 1999-current Flags for 27 common conditions Includes the CCW chronic conditions previously found in the BASF
MBSF: National Death Index (NDI) 2012 NA 1999-2016 Date of death Includes new variables not previously offered
Cause of death
State of death
MBSF: Other Chronic or Potentially Disabling Conditions 2015 NA 1999-current Flags for 35 additional chronic health, mental health, substance abuse, and potentially disabling condition categories beyond the chronic conditions flags above Includes new chronic conditions not previously offered
Table 1. List of Medicare Beneficiary Eligibility and Enrollment Files Since 1999