Study population
The data used in this study were from the survey on FDCS that conducted in Shandong province, China in 2018. Shandong is the second most populous province in China with more than 100 million population in 2018, of which about 40% lived in rural areas. The multi-stage random sampling method was used to select the subjects. Firstly, we randomly selected three cities (Liaocheng, Zibo, and Binzhou) as study sites. Secondly, two counties were randomly selected from each sample city. Thirdly, five townships were randomly selected from each country and then six villages from each selected township were randomly selected. Finally, 16 households were randomly selected from each sample village, and only one member of a family (generally the main income earner or the elderly) was reviewed. Strict random number table sampling was used throughout the study. A total of 2,979 respondents were interviewed, of whom 1,210 patients with chronic diseases (hypertension, coronary heart disease, or diabetes) were included in the analysis.
The investigators were recruited from Shandong University. They were strictly trained before the investigation, including understanding the principles and methods of the survey, and standardizing the definition and interview skills of each study indicator, with the purpose of ensuring the quality of the survey. After the training, the investigators were given test on training effectiveness, and only those who were qualified could participate in the formal investigation. Each sampled township was also supervised by a trained supervisor who was responsible for guiding and (logical) checking survey questionnaire to ensure the accuracy and completeness of the questionnaires.
Measures
Family doctor contract services
The status of FDCS was a two-category variable. It was measured by the following yes-no question: “Did you contract with the family doctors this year?” [16, 17]
Similar with previous studies, household income, educational attainment, and employment status were used as proxies for SES [18-20]. In this study, we tried to use the main determinants of SES (i.e. household income, educational attainment and employment status). Educational attainment was recoded into three categories: low (primary education or below), intermediate (junior education), and high (senior education or above). According to the quartile methods, household income was classified into four categories: Q1, Q2, Q3, and Q4, from lowest to highest. We recoded employment status into two categories: unemployed and employed. Specifically, lower educational attainment, lower quartile, and unemployed status represent lower SES.
Health-related quality of life
Health-related quality of life (HRQOL) was assessed using the EQ-5D questionnaire, which consists of 5 health dimensions (mobility, self-care, usual activities, pain/discomfort, and anxiety/depression). Each of the five dimensions has three levels indicating no problems, moderate problems or severe problems [21]. The EQ-5D-3L utility values are generated by weighting each dimension of the HRQOL, which use the time trade-off model set for the Chinese general population [22], ranging from -0.149 to 1.0, and the higher EQ-5D-3L utility values represent higher HRQOL.
Covariate variable
The marital status was divided into three categories: single, married, and others (divorced or widowed). The number of family members was divided into three categories: two and less, three to four, and more than four. The drinking status was divided into three categories, including never drinkers, current drinkers, and former drinkers. Other covariate variables include age, gender, multiple chronic diseases status, and physical exercise.
Statistical analyses
Statistical analyses were performed using Stata 14.0. The reported credible intervals (CIs) were calculated at the 95% level and P values less than 0.05 were considered statistically significant. First, we used frequencies and percentages to describe the demographic characteristics of the respondents by family doctors contracting status (Yes/No). Second, independent-samples t-test was used to compare the EQ-5D-3L utility values between contracting status. Third, Tobit regression was employed to explore the association between contracting status of family doctors and EQ-5D-3L utility values. Logistic regression was preformed to explore the association between contracting status and each dimension of EQ-5D-3L. Finally, in order to explore whether there were differences in the association among different SES status (income, educational attainment, and employment status), interaction terms between contracting status and SES status were introduced in the Tobit regression models, and if the interaction terms were statistically significant in that Tobit regression model, we further stratified the regression analyze by different SES status (income, educational attainment, and employment status).