Data came from the 2010–2013 NHIS. Details about the sampling frame and sample design have been published previously (Parsons et al. 2014). Interviews were conducted in respondents’ homes, with some telephone follow-ups. One person from each family answered questions about family members’ age, health insurance coverage, and general health status. One randomly selected “sample adult” aged 18 years and older from each family answered questions about himself or herself. In each year, the final response rate among sample adults was over 60 percent and data were obtained from approximately 30,000 sample adults. Details of the sample sizes and response rates are available in the survey documentation for each relevant year (e.g. NCHS 2013).

At the conclusion of the sample adult interview, respondents were asked: “To help us link your survey data with vital statistics and health-related records of other government agencies, we would like the last four digits of your Social Security Number. The National Center for Health Statistics uses this information for research purposes only. Providing this information is voluntary. Federal laws authorize us to ask for this information and require us to keep it strictly private. There will be no effect on your benefits if you do not provide this information. What are the last four digits of your Social Security Number?” Respondents were similarly asked for the last four digits of the HICNUM if they were Medicare eligible. Respondents who answered “no,” “refused,” “don’t know,” or “don’t have a SSN (or Medicare number),” were asked: “May we try to link your survey data without [your SSN/HICNUM]?”

The questions were the same whether administered in-person or by telephone. For the analysis, record-linkage consent was defined as permission to use the SSN or HICNUM to link, or permission to link without either an SSN or HICNUM. Record-linkage refusal was defined as refusal to provide either the SSN or HICNUM combined with refusal to allow linkage without the SSN or HICNUM. For the 2010–2013 NHIS, unweighted linkage refusal ranged from 10.0 percent in 2010 to 11.8 percent in 2013.

The selected health conditions examined were hypertension, heart disease, diabetes, cancer, chronic obstructive pulmonary disease (COPD), serious psychological distress (SPD), and stroke. Hypertension was defined as having been told by a doctor or other health professional of having hypertension on two or more different visits. Heart disease was based on responses to questions about having ever been told by a doctor or other health professional of having coronary heart disease, angina (angina pectoris), a heart attack (myocardial infarction) or any heart disease or condition. Diabetes (excluding borderline diabetes) and stroke were also based on “yes” or “no” responses to questions about having ever been told by a doctor or other health professional of having these conditions. Cancer was based on responses to having ever been told by a doctor or other health professional of having cancer or a malignancy excluding nonmelanoma skin cancer. COPD was based on positive responses to questions about having ever been told by a doctor or other health professional of having emphysema or having been told in the past 12 months of having had chronic bronchitis (American Thoracic Society 2004). SPD was measured using the K6, a series of six psychological distress questions asking how often a respondent experienced symptoms of psychological distress during the past 30 days (Kessler et al. 2003). Small numbers of respondents were missing data: hypertension (n=253); heart disease (n=54); diabetes (n=91); cancer (n=106); COPD (n=44); SPD (n=2,025); stroke (n=121) respondent-assessed health status (n=66); and total number of conditions (n=2,534). Records with missing data were not included in the health-outcome specific analyses but were retained in the overall study population.

Demographic characteristics included race and ethnicity, age, and poverty. Race and ethnicity were categorized as Hispanic, non-Hispanic White, non-Hispanic Black and all other races/ethnicities. Age groups included 18–44 years, 45–64 years, and 65 years and over. Annual family income was categorized into a poverty index ratio (PIR) of below 100 percent of the federal poverty level, 100 percent–199 percent of the federal poverty level, 200 percent–399 percent of the federal poverty level, and 400 percent or more of the federal poverty level. Imputation was used to assign a poverty level for records with missing income data (Schenker et al. 2009). Because Miller et al. found a possible association with imputation status and record-linkage refusal (Miller et al. 2011), imputation status was included as an independent variable, with categorical responses: providing all income information, providing income by categories, and providing no income information.

Point estimates, corresponding variances, and 95 percent confidence intervals were calculated using SAS callable SUDAAN software (SUDAAN 2008) to provide weighted estimates and account for the complex sample design. Overall associations for categorical variables were evaluated using the Rao Scott Chi Square statistic for weighted survey data.

We used multivariable logistic regression models that included one health condition or characteristic and controlled for age group, sex, race/ethnicity, PIR, and income imputation status. Trend tests identified linear relationships between linkage refusal, health status, and total number of health conditions.