2.1 Setting and sample
Jinzhai County located in the western region of Anhui Province, which situated in Eastern China[20]. There are 68 thousand people and 3814 km2 of total area in Jinzhai County. Jinzhai County is an important source of the Chinese Revolution and the birthplace of the people's army. However, Jinzhai County is one of the first state-level key poverty-stricken counties, with a poverty rate of 11.2%[21].
The data in this article came from the first Non-communicable and Chronic Disease Risk Factor Surveillance (NCD Surveillance) in Jinzhai County, 2017, which was launched by Jinzhai County Health and Family Planning commission, Lu'an Municipal Center for Disease Control and Prevention and Jinzhai County Center for Disease Control and Prevention. The multi-stage stratified cluster random sampling was used to sample rural adult residents in Jinzhai County. Firstly, Probability Proportionate to Size (PPS) sampling was used to randomly select six towns from Jinzhai County which had 23 towns or townships. Then, five administrative villages were randomly selected by PPS sampling from the above each town which had already been selected. Thirdly, simple random sampling was used to randomly select two village groups from the each selected administrative village. Fourthly, 45 residential households were randomly selected from each village group by cluster random sampling. Finally, one permanent resident aged 18 or above in each residential household was randomly selected to participate in this survey by Kish Grid Sampling.
The sample size was calculated according to the following calculation formula, N = deff*μ2*p(1-p)/d2, which α = 0.05, μ = 1.96, p = 9.7% (Data of this study from NCD Surveillance, so value of p adopted the prevalence of diabetes in China[22]), design effect (deff) = 1.5,relative error (RE) =15%,permissible error (d) = RE*P = 15%*9.7%. By the calculation, N ≈ 2385. If considering the rate of rejecting accept investigation was 10%, the final sample size N = 2623.5 ≈ 2700, which could assure adequate power to detect statistical significance.
2.2 Questionnaire survey
2.2.1 Demographic and socioeconomic data
The uniform questionnaire in Non-communicable and Chronic Disease Risk Factor Surveillance in China[23] was used to collect the demographic and socioeconomic information, including gender (males, females), age (years old), occupation (agriculture and forestry, professionals, industrial and commercial service, housework, others), marital status (single, married / cohabitation, Divorced / widowed / separated), education (primary or below, junior school, senior or above) and annual family income (RMB).
2.2.2 Self-reported data of height, weight and waist circumference
The self-reported data of height, weight and waist circumference were collected before the actual measurement. The participants were asked: “Do you know your height?” “Do you know your weight?” “Do you know your waist circumference?” Once participants answered that they don’t know, jump to the next question. If the participants answered “Yes”, the self-reported data will be collected with the question: “What is your height?” “What is your current weight?” “What is your current waist circumference?” In order to avoid misunderstand to the null value of self-reported height, weight and WC which may mean missing or unknown, each questionnaire was checked carefully.
2.3 Body measurement
Body measurement of height, weight and waist circumference was conducted by trained fieldworkers from the NCD Surveillance. Measurement of and weight and WC was conducted on an empty stomach in the morning, which the respondents had been informed one day in advance. The physical examinations were completed by two investigators. Height and weight were measured with an accuracy of 0.1 cm and 0.1 kg, respectively. WC was measured at the iliac crest with an accuracy of 0.1cm. In order to improve the accuracy of measurement, the participants should remove shoes and heavy clothing when measured the height and weight; and the waist should be completely exposed when measured the WC.
2.4 Definition of index
2.4.1 Classification of BMI
BMI was calculated by the calculation formula: BMI = weight (kg) / height 2 (m) and used both self-reported (namely self-reported BMI) and measured (namely measured BMI) data. Classification of BMI was according to the Chinese adult classification system[24, 25]: underweight (BMI < 18.5 kg/m2); normal weight (18.5 ≤ BMI < 24.0 kg/m2); overweight (24.0 ≤ BMI < 28.0 kg/m2); and obesity (BMI ≥ 28 kg/m2).
2.4.2 Classification of increased WC
Classification of WC was according to the sex-specific cut-offs for Chinese adults[25, 26]: normal WC or increased WC (WC ≥ 85 cm for men and WC ≥ 80 cm for women).
2.4.3 Difference value between self-reported and physical measured
The Difference value (D) was calculated by subtracting the physical measured data from the self-reported data. Therefore, a negative value indicates the participants under-reported the data and a positive value indicates they over-reported the data.
2.5 Statistical Methods
The double entry and validation of the data used Epi Data 3.1 software (Epi Data foreningen [Epi Data Association], Odense, Denmark), and the data analysis used SPSS 10.01 (SPSS Inc., Chicago, IL, USA). The awareness rate was described by rate (%) and 95%CI. A chi-squared test was used to compare the awareness rate among the different groups. The accuracy of self-reported was described by the mean difference value (D = self-reported data minus physical measured data) and 95%CI. Paired Sample t test was used to compare the self-reported and measured data between the different groups. The agreement between self-reported data and measured data was analyzed by the intraclass correlations (ICCs). In addition, Bland-Altman plots with limits of agreements (LOAs), which gives the mean and 95%CI of the difference between self-reported values and measured, was also used to assess the concordance[27]. The impact of self-report on categorization of BMI (underweight, normal weight, overweight, obesity) and categorization of WC (normal WC and increased WC) were also assessed using Cohen’s kappa and 95%CI. The criteria of Landis and Koch [28] was used, namely < 0 is poor; 0 - 0.20 is slight; 0.21- 0.40 is fair; 0.41 - 0.60 is moderate; 0.61-0.80 is substantial; and 0.81- 1.00 is almost perfect. The effect of demographic and socioeconomic variables affected the rate of awareness for height, weight and WC were analyzed by multi-factor Logistics regression, which used the variable of aware or not aware (0 = yes, 1 = no) as the dependent variable. The effect of self-report accuracy for height, weight and WC were analyzed by multiple linear regression, which used the difference value (D = self-reported data minus physical measured data) as the dependent variable. The p-values < 0.05 was considered to be significant.
2.6 Ethical aspects
The need for ethics approval is deemed unnecessary, because the Non-communicable and Chronic Disease Risk Factor Surveillance (NCD Surveillance) is a responsibility for the government department to know the chronic diseases and the risk factors in the local according to the government document[29]. Each participants signed the informed consent before the survey.