Sociodemographic and outcome characteristics of the 968 participants with arthritis are presented in . The sample was on average 57 (±13) years old, with an average BMI of 30 kg/m2 (±7), and 2 (±2) comorbid conditions. The majority were female (74%), non-Hispanic white (76%), educated (52% beyond HS), with a household income less than $45,000 (66%). About half of the participants (49%) reported non-professional occupations that typically would be physically demanding (e.g., service industries, farming, manufacturing). Homeownership was reported by 78% of the participants.
Participant Sociodemographic and Outcome Characteristics*
For each physical health status outcome assessed, the participants reported a mean PCS score of 40.74 (±12.12) and mean HAQ score of 0.67 (±0.73). On average, they reported an average of 8.6 (±10.65) unhealthy physical days and 9.2 (±10.32) limited activity days per month on the CDC HRQOL healthy days scale.
Also presented in are the sociodemographic characteristics stratified by non-Hispanic white (white) and non-Hispanic black (black) racial subgroups, with p-values reported for each test to indicate substantial differences by race. Results show that blacks are more likely than whites to have lower education, to be non-homeowners, to have a non-professional, physically demanding occupation, to have lower household income, and to live in a community with high poverty rate. Blacks had greater disability than their white counterparts as noted with higher HAQ scores. While there were no significant differences by race for PCS, number of physically unhealthy days or number of limited activity days, there was a trend toward lower PCS scores for blacks.
Correlation analyses (not shown) revealed that individual and community SES variables were weakly correlated with each other (ranging from 0.10 to 0.40, p<0.01), and also weakly correlated with the physical health outcomes (ranging from 0.06 to 0.36, p<0.01). Correlation analyses between the physical health outcomes indicates that they are moderately correlated with each other (ranging from 0.55 to 0.73, p<0.001).
We examined the independent effect of each SES variable alone or in concert, on the physical health status outcomes. A staged set of models was developed to allow for in-depth examination, with results of these analyses presented in , , and . Since the referent is conceptualized as the most advantaged class in all cases, the PCS (higher score implies better health) tends to have negative parameter estimates for the SES variables, while the other outcomes (higher scores imply poorer health) tend to have positive estimates.
Parameter estimates for each of five SES variables singly with four Physical Health Outcomesa, B (95% Confidence Interval), p-value
Parameter estimates for single of four individual SES paired with Community SES with four Physical Health Outcomesa, B (95% Confidence Interval), p-value
Parameter estimates for all five SES variables cumulatively in blocks arranged according to level of influence, B (95% Confidence Interval), p-value
Each of the five individual-level SES variables was entered singly into models for each of the four outcomes, adjusting for age, gender, BMI, race and comorbid condition count. Most of the resulting estimates () are significantly different from zero, with most being p<0.01. The effect of education on each of the physical health outcomes is shown in . People with less than a high school education scored about 4.8 points lower on the PCS compared to those with education beyond high school. They also reported about 3.6 more days per month of poor physical health and 3.5 more days of limited activity related to health. Finally, participants with less than a high school education scored 0.24 higher on the HAQ scale of disability, indicating greater disability.
While the significant role of educational attainment, occupation, and homeownership is observed for all physical health outcomes, household income of <$15,000 has the largest negative effects on PCS and physically unhealthy days outcomes. The role of community poverty is not as strongly associated with worse physical functioning (PCS) as are the individual-level SES markers, however high community poverty is significantly associated with greater number of physically unhealthy days and HAQ disability, with a trend for greater limited activity days.
The relationship between each of the four individual SES measures (education, household income, occupation, and homeownership) and physical health outcomes was then examined in the context of community poverty, adjusting for covariates (). The first set of models examined educational attainment as the individual level SES measure and community poverty level on physical health outcomes. Individuals with less than a high school degree had significantly poorer scores on all four health status outcomes compared to individuals with greater than a high school degree for physical functioning (β=-4.89, p=≤0.001), physically unhealthy days (β =3.53, p=≤0.001), limited activity days (β =3.50, p=0.036), and HAQ disability (β =0.25, p=0.005). Individuals with a high school degree had significantly poorer PCS and physically unhealthy days, as well as a trend for poorer HAQ score compared to those with greater than a high school degree. In two models, living in the poorest communities (i.e., highest community poverty rate) was associated with poorer health status outcomes independent of education level. The high poverty group had a statistically significant negative impact on HAQ disability scores (β =0.08, p=0.029) and a trend for greater physically unhealthy days (β =1.10, p=0.055). Comparing these results to those in , we observed a similar educational attainment effect, but a reduced community poverty effect. Educational attainment appears to more strongly explain all physical health outcomes, although both remain significant for HAQ disability.
This dynamic is observed in the occupation and homeownership sets of regression models as well. When compared to the referent groups, having a physically demanding job and not owning a home both were associated with poorer physical health outcomes. As before, when these groups were individually adjusted for community level poverty, poorer HAQ disability scores and greater numbers of physically unhealthy days were seen in the highest level poverty groups. Comparing to results shown in , we see that the effects of occupation and homeownership seem to remain independent and decrease only slightly.
Most striking are those models including both household income and community poverty (). The effects for household income approximate those given in and remain significant, while the significant effects for community poverty are eliminated. The information provided by community poverty is apparently subsumed by the individual income and no independent association with any physical health outcome remains. When compared to those making more than $45,000, participants earning less than $15,000 had lower PCS scores by 8.56 points (p=≤0.001) and higher HAQ disability scores by nearly half a point (β =0.48, p=≤0.001). Additionally, those with lowest household income had increased number of physically unhealthy days, 5.78 days more per month (p=≤0.001) and increased limited activity days, 7.24 days more per month (p=≤0.001). Lesser, but significant (p<0.01) results were seen for results comparing participants from households earning $15,000-$45,000 to the over $45,000 income group.
Finally, a series of models successively adds SES variables until all five SES variables are in each model (see ). Considering both the relative sizes of parameter estimates and their p-values for each outcome in and , the order of the introduction of SES variables has been arranged so the variables that tended to be less influential are entered first (i.e., occupation, homeownership), community poverty next, and then education; the final variable entered is household income.
The Block 1 models have occupation and homeownership entered together and they are independently significant for PCS and HAQ. Community poverty was added in the second set of models (Block 2), and living in the poorest community is associated with greater number of physically unhealthy days (β =1.09, p=0.046) and greater HAQ disability score (β =0.08, p=0.027). Moreover, significant independent effects are maintained in occupation and homeownership. In Block 3, educational status is included in the models. While the effects for less than high school are significant, homeownership and high community poverty also show independent significance for some of the outcomes. However the significant independent effect of occupation is eliminated with the addition of education into this model.
Block 4 adds household income to the other variables to create models that contain all the SES variables. Income is significant at both levels (<$15,000 and $15,000-$45,000) relative to the >$45000 level, while all other SES variables lose significance, with the exception of HS education on PCS, where it remains a significant individual-level SES variable.