BASE Enrollment File

The Centers for Medicare & Medicaid Services (CMS) maintains an annual Master Beneficiary Summary File (MBSF) that includes all Medicare beneficiaries. In SEER-Medicare the MBSF is limited to persons who were found in both the SEER and Medicare data. The MBSF has multiple segments, as briefly described below. A more detailed explanation can be obtained hereExternal Web Site Policy from the CMS’s Chronic Conditions Data Warehouse (CCW). In SEER-Medicare terminology, “BASE” is used to distinguish data files that come directly from CMS and serve as the basis for summarized Condensed Resource (CoRe) files and the SEER-Medicare specialized SEER*stat databases.

  1. Base (A/B/C/D) segmentExternal 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). Part C and Part D contract and plan identifiers are included.
    • 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. Chronic Condition Flag Files - CMS created claims-based algorithms, which are available on the Chronic Conditions Warehouse (CCW) website, to summarize the presence of common conditions. Because the summarized indicators (“flags”) for the presence of each condition are determined using claims data it is not possible to ascertain this information for beneficiaries enrolled in managed care/HMOs. This limitation also applies to newly-eligible Medicare beneficiaries who may have only a partial year of FFS coverage. Thus, in order for the flag to indicate the presence of a condition, the claims for the beneficiary must indicate treatment for that condition (i.e., the condition must be listed as a diagnosis on the service claim) and the beneficiary must also have had continuous Part A/B-FFS coverage during the specified time period. It is important to note that the major objective for creating the flags was to allow for a quick, initial identification and extraction of beneficiaries with a given condition from the larger Medicare population. The flag definitions were intended to be broad, so that researchers could extract the data based on the flag definitions and then refine their specifications as needed for their specific analyses. The condition definitions were not intended to calculate population statistics.

    There are two versions of the CCW Chronic Conditions Flag Files because the conditions assessed and/or the diagnosis codes included in the condition algorithms have changed over time. These annual files contain first occurrence date and end year flag to indicate the presence or absence of each assessed condition.

    27 CCW Chronic Conditions File DocumentationExternal Web Site Policy indicates the presence or absence of 27 conditions and is available beginning in 1999; 2021 is the last planned year for this file. These annual files span the transition from ICD-9 to ICD-10; therefore, the condition algorithms consider both ICD-9 and ICD-10 diagnosis codes.

    30 CCW Chronic Conditions File DocumentationExternal Web Site Policy indicates the presence or absence of 30 conditions and is available beginning in 2017. These annual files all occur after the transition from ICD-9 to ICD-10; therefore, the condition algorithms consider only ICD-10 diagnosis codes.

  3. Other Chronic or Potentially Disabling Conditions documentationExternal Web Site Policy contains first ever occurrence date and end year occurrence date for an additional 35 chronic or potentially disabling conditions not included in the above chronic conditions segment (e.g., mental health; tobacco, alcohol and drug use; developmental disorders; disability related conditions; behavioral health conditions); claims-based algorithms are available on the Chronic Conditions WarehouseExternal Web Site Policy website. Again, these flags are determined based on Medicare services provided beginning in 1999; therefore, the same considerations outlined above pertaining to the chronic condition flags should be taken with these flags.
  4. 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.

Given there are two cohorts of persons included in the SEER-Medicare data (persons with and without cancer), there are two subsets of MBSF data available via SEER-Medicare (MBSF-Cancer and MBSF-Control); the file documentation is the same. The MBSF-Control can be used to identify controls for the persons with cancer or to develop population-based estimates of health care utilization, e.g. use of cancer tests such as PSA and mammography in the entire population. The persons in the MBSF-Control file have been identified from a random 5% sample of Medicare beneficiaries residing in the SEER areas. If a person in the 5% sample is diagnosed with cancer they are removed from the MBSF-Control file and will only appear in the MBSF-Cancer file. Like all persons with cancer reported to a SEER registry, their cancer information will be included in the SEER-Medicare Cancer File with a flag to indicate that they are in the 5% sample. Additionally, these persons will be included in the SEER-Medicare 5% Cancer File, which is a subset of the SEER-Medicare Cancer File (e.g., restricted to cancer diagnoses only among persons included in the 5% sample).

Last Updated: 20 May, 2024