README for Medicare 2014-2018 feasibility files

NCHS-CMS Medicare 2014-2018 feasibility files were originally posted in May 2021.
In October 2021, the original files were replaced with revised files that corrected the match status variable (MEDICARE_MATCH_1418) for less than 1% of survey participants (SPs) across all surveys.  

The match status variable (MEDICARE_MATCH_1418) has 3 allowable values:
1= Linkage-eligible and linked to one or more years of CMS Medicare administrative data CMS Medicare Match Status (2014-2018) 
2= Linkage-eligible and not linked to any year of the CMS Medicare administrative data 
9= Ineligible for CMS linkage or under age 18

There were two types of changes.
1. Change from 2 ("not linked") to 9 ("ineligible or under 18")  
As described in the Linkage Methods and Analytic Guidelines, only NCHS survey participants who have provided consent as well as the necessary personally identifiable information (PII) are considered linkage-eligible.
The original match status variable mistakenly categorized some SPs who provided consent for linkage but did not provide adequate PII as "eligible and not linked." In the revised files, their match status is coded as 9.

2. Change from 9 ("ineligible or under 18") to 2 ("not linked") 
All child SPs were coded as "9" for the public-use feasibility files. (The match status variable for child SPs is a restricted-use variable that may be accessed through the NCHS Research Data Center under an approved project proposal.) 
The original match status variable miscoded a small number of SPs as children instead of adults, so their match status was coded as "9". In the revised file, these SPs are coded as "2."

The revisions only changed the match status variable. There were no changes to any of the indicator variables for whether each participant has linked record(s) on the different types of Medicare files.

Note that there should not be an impact on researchers’ project feasibility assessments since there were no changes for survey participants who matched to the Medicare data (MEDICARE_MATCH_1418=1). 
However, researchers using the match status variable to determine linkage eligibility and create adjusted survey weights are advised to use the revised file.