CARDIA is a prospective, multi-center investigation of contributors to changes in cardiovascular disease risk factors and cardiovascular disease onset and progression during the transition from young adulthood to middle age. In 1985–1986, 5,115 Blacks and Whites aged 18–30 years were recruited and examined in field centers in four locations: Birmingham, Alabama; Chicago, Illinois; Minneapolis, Minnesota; and Oakland, California.
10 (link), 11 (link) Participants were re-examined 2, 5, 7, 10, 15, 20, and 25 years later, with high retention rates (91%, 86%, 81%, 79%, 74%, 72%, and 72% of the surviving cohort, respectively). The current study uses data from all the exam years in which participant addresses were geocoded: baseline, 7, 10, 15, 20, and 25 year examinations. Given the unique historical context of residential segregation of Blacks from Whites in the US, and the nearly non-overlapping levels of segregation scores in Black and White CARDIA participants, the current study is limited to Black participants. Of the 2,637 Black baseline participants, 313 were excluded for not having two measures of blood pressure and 44 were excluding for missing data on baseline covariates, leaving a final sample of 2,280. The CARDIA study was approved by the institutional review boards at all study sites and all participants provided written informed consent.
Neighborhood-level racial residential segregation of Blacks from other racial/ethnic groups was measured at each available exam using the local G
i* statistic,
12 based on the geocoded addresses of CARDIA participants linked to the nearest US Census: 1980 for baseline, 1990 for years 7 and 10, 2000 for years 15 and 20, and 2010 for year 25. The G
i* statistic returns a Z-score for each neighborhood (census tract), indicating the number of standard deviations the racial composition (in this case, percent Black) in the focal tract and neighboring tracts is from the mean racial composition of some larger areal unit surrounding the tract (in this case the surrounding metropolitan area or county). See the
online appendix for further details.
Segregation was modeled continuously for the analyses of all Black participants. Segregation was categorized as high (G
i* >1.96), medium (G
i* 0 – 1.96), and low (G
i* < 0) for the analyses in the subset of participants in the high segregation category at baseline. The cutpoints for the categories were chosen to be consistent with the critical Z-score values for a 95% confidence interval (−1.96 and 1.96), which corresponds to statistical significance at the α=0.05 level.
13 (link) No participants lived in areas with G
i* < −1.96, so we used a cutpoint of 0 for the low category to indicate a Z-score equal to or below the mean racial composition of the surrounding metropolitan area/county.
Blood pressure was measured at each CARDIA examination by trained technicians using either a random zero mercury sphygmomanometer (baseline – year 15) or an oscillometer (years 20–25). Oscillometer readings have been calibrated to the sphygmomanometric measures.
14 (link) Resting systolic and diastolic blood pressure was measured 3 times at 1-minute intervals; the average of the second and third measurements was used in the analyses. We accounted for treatment effects by adding 10 mm Hg to the observed systolic blood pressure and 5 mm Hg to the observed diastolic blood pressure in treated subjects.
15 (link) We also conducted a sensitivity analysis where we adjusted for self-reported medication use as a covariate instead.
Several covariates associated with residential segregation and blood pressure were adjusted for in our analyses as potential confounders or mediators including age, sex, current marital status (dichotomized as married or co-habiting vs. not married/co-habiting), education (categorized as high school graduate or less, some college, and college degree or higher, neighborhood poverty (percent of neighborhood residents who were below the U.S. Census Bureau-defined poverty threshold), and neighborhood population density. Individual income data were not collected at baseline, so we did not include it in our primary analyses. As a sensitivity analyses, we ran all models adjusting for time-varying income using data from the year 5 examination for baseline income.
Cigarette smoking was dichotomized as current vs. not current. Leisure-time physical activity was assessed with the CARDIA physical activity questionnaire, which includes questions on the frequency of participation in 13 categories of sports and exercise during the previous 12 months.
16 (link) A score was summed across all activities and expressed continuously in Exercise Units. The reliability and validity of the instrument are comparable to other activity questionnaires.
17 (link), 18 Body mass index (BMI) was calculated as weight (in kg) divided by height (in m
2). Body weight in light clothing was measured to the nearest 0.5 pound using a balance beam scale. Height without shoes was measured to the nearest 0.5 cm using a vertically mounted centimeter ruler and a metal carpenter’s square.
We calculated descriptive statistics for study variables by examination year and segregation category (high and medium/low). Segregation scores can change over time if participants move to neighborhoods with different levels of segregation or if participants stay in the same place and the level of segregation changes in their neighborhood. To get a better sense of what drove changes in segregation score, we used a paired t-test to calculate the mean within-person change in segregation score between each successive exam separately for movers (those who changed census tracts between exams) and stayers (those who did not). Fixed-effects regression models were used to estimate associations of within-person changes in segregation with within-person changes in blood pressure.
19 Fixed-effects models focus only on within-person variation rather than between-person variation. This approach has the advantage over mixed-effects models in that it tightly controls for all measured and unmeasured time-invariant characteristics. In fixed-effects models, characteristics that do not change over time are conditioned out of the estimation process. For this reason, fixed-effects models cannot be used to examine main effects of time-invariant characteristics. However interactions involving these variables can be examined. Baseline time-invariant covariates including age, sex, and field center were tested for interactions with time to allow for different trends in blood pressure over time associated with these characteristics. All statistically significant interactions (α=0.05) were retained in the models.
Our first set of multivariable regression models were designed to assess the overall relationship between within-person changes in neighborhood-level racial residential segregation and within-person changes in blood pressure over the follow-up period. For these analyses, segregation was modeled continuously using the full sample of black participants. We also assessed whether this relationship varied by participant age, neighborhood poverty, education, and income by testing interaction terms between these covariates and segregation. To investigate whether reductions in segregation were associated with reductions in blood pressure among residents who were in highly segregated neighborhoods at baseline, we also examined this relationship among the 1,861 Black participants living in high segregation neighborhoods at baseline (81.6% of total sample). For these analyses, segregation was modeled categorically in order to estimate changes associated with more meaningful changes in levels of segregation. As a sensitivity analyses, we ran this same model excluding participants who changed to lower segregation neighborhoods and changed back to high segregation neighborhoods at any subsequent exam. All analyses were conducted using SAS 9.4 (Cary, NC).
Kershaw K.N., Robinson W.R., Gordon-Larsen P., Hicken M.T., Goff DC J.r., Carnethon M.R., Kiefe C.I., Sidney S, & Diez Roux A.V. (2017). Association of Changes in Neighborhood-Level Racial Residential Segregation With Changes in Blood Pressure Among Black Adults: The CARDIA Study. JAMA Internal Medicine, 177(7), 996-1002.