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Results
We submitted 1 036 449 people to NDI. Follow-up time from the date of last contact was 1–13 years with a median of 6 years and average of 6.3 years. However, since we had 69% missing self-reported race and ethnicity (online supplemental table 1), we imputed race and ethnicity resulting in populations that were 9% A/PI, 31% black, 13% Hispanic, 2% AI/AN/multiracial, 45% white and 0.3% not reported due to a missing address (table 1).
Characteristics of the population submitted to NDI (n=1 036 449) and the population with scores above the cut-off (presumed dead) (n=38 862) and scores below the cut-off (presumed alive) (n=334 003)
Of the 1 033 477 people, with imputed race information, we found 143 452 (13.8%) were missing SSN. Percent missing SSN significantly differed among race and ethnicity (χ2, p<0.001), and a race–sex-adjusted logistic regression analysis showed all race and ethnicity categories were significant compared with the white population as a reference group, with Other having the largest OR (OR=14.3 (11.9, 17.1)) followed by the Hispanic population (OR=2.34 (2.30, 2.39)) and by the A/PI population (OR=1.56 (1.53, 1.60)). An analysis of self-reported data showed similar results, except that the Other group had a smaller OR (online supplemental tables 2 and 3).
We obtained summary statistics on 372 865 people that matched records in the NDI (figure 1). We found 663 061 people did not match the NDI database and 522 people were rejected for inclusion of special characters in the fields and one duplicate record was removed. Therefore, we classified 663 583 as alive according to NDI. However, there were 1188 deaths among these patients according to the VDW sources by 31 December 2017.
KPMAS submission matches the National Death Index and the process of inclusion. KPMAS, Kaiser Permanente Mid-Atlantic States.
Above PS cut-off (presumed dead)
Of the 372 865 people that matched records in NDI, we found 38 862 unique people had PSs above the class cut-off threshold (tables 2 and 3, figure 1).
Exact match on submitted variables for records with scores above the cut-off (presumed dead)
NDI matches (n=372 865) stratified by class and probabilistic score, with death dates compared with the VDW
There were no records in Class 1 because state of birth was not submitted. Class 3 had very few matches because father’s surname and race were not submitted and very few people submitted had marital status information. Table 2 shows that there was a high percentage of exact matches between the data submitted and the NDI database for each field within each class. Only middle initial and state of residence had less than 95% matching for all classes. A high percentage of above PS cut-off records (89%) were matched exactly by their 9-digit SSN.
Below PS cut-off (presumed alive)
Of the 372 865 people that matched records in NDI, we found 334 003 had PSs that were below the threshold for class, therefore considered poor matches and the KPMAS member was presumed alive (table 3). Most, (82.7%) were in Class 5 (people who had an SSN that did not match any SSN in NDI).
Comparison between NDI and the KPMAS VDW on death date
Comparing death dates more closely, we analysed the number of NDI records above the PS cut-off, where the death dates matched exactly with the VDW, were missing in the VDW or did not match the VDW (table 3, figure 1).
Of the 38 862 above PS cut-off records, there were 1539 (4.0%) records where NDI and VDW matched in deceased vital status but had non-matching death dates. The death dates differed with a median of 2 days, IQR (1–9 days), maximum of 10 212 days. Of these, there were 1421 that also matched exactly on first and last names, and birth month, and birth year and sex and 1021 that matched on all variables.
For the below PS cut-off records (table 3), we analysed possible matching on 1858 death dates between the VDW and NDI, due to the possibility that some of these NDI records were true matches and therefore could be linked to identifiable deaths and cause of death could be obtained. We found 870 records where death dates matched exactly to our VDW. Of these, 300 also matched exactly on first and last names, and birth month, and birth year and sex; there were only 17 that mismatched on day of birth but 139 had mismatches on state of residence.
Sensitivity and specificity
The submission to NDI provided an additional 10 017 records presumed dead compared with using our VDW alone at the time of submission (tables 3 and 4). Table 4 shows a comparison of deaths found with the NDI best score algorithm to the deaths found in the VDW for all records submitted. We had 74.2% sensitivity and 99.7% specificity on death matches, with NDI as the gold standard.
NDI matches compared with VDW deaths by 31 December 2017 for complete submission
PSs stratified by sex and race and ethnicity
The above cut-off scores had a higher percentage of men (55.7%) compared with below cut-off scores (52.0%), p<0.0001. Imputed race and ethnicity were significantly different between above and below cut-off scores, p<0.0001: above scores had lower percentages of A/PI and Hispanic people and had higher percentages of white and black people (table 1). Similar results were seen for self-reported race (online supplemental table 1).
Class 4 matches are independent of SSN and dependent on name matching. In multivariate linear regression analyses of Class 4 scores for above and below the PS cut-off, we found sex and imputed race and ethnicity were significantly associated with PS in each group. In the above cut-off-score group, women had slightly higher scores compared with men (online supplemental table 4). Compared with the reference white population, the imputed A/PI populations group had 4.8 times higher PS, while scores for Hispanic people were not significantly different; similar results were seen for self-reported race (online supplemental table 4). In the below cut-off-score group, women had lower PS compared with men. The imputed Hispanic and black populations had higher PS while the A/PI group and the other group had lower PS compared with the reference white population, both statistically significant; similar results were seen for self-reported race, except that the black and other group did not have statistically different PS scores compared with the white population (online supplemental table 4).
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