Definitions and measurement of variables
Information on NCD risk factors was collected through self-reports. The survey questionnaire was adapted from the WHO STEPS and WHO Study on Global Ageing and Adult Health (SAGE) instruments (Additional file
2 questionnaire). Respondents were asked questions on tobacco use, alcohol consumption, physical activity and fruit and vegetable consumption. All NCD risk factors were dichotomised [
24] to indicate whether respondents reported or did not report any NCD risk factors. For tobacco use respondents were asked whether they currently smoke tobacco products, yes=1 and no=0. Alcohol consumption was measured based on the intensity of alcohol consumed in the past 30 days. Respondents were asked (only those who reported to have consumed alcohol in the past 30 days) about the number of standard alcohol drinks they had each day in the past 7 days and if those who reported to have had three or more drinks per day (of approximately 60 g alcoho1) were considered to have exercisive drinking=1 and 0=otherwise.
An adapted version of the WHO STEPS questionnaire for adults was used to assess participants’ self-reported physical activity [
25]. The questionnaire assessed physical activity done in the past 7 days in the domains of work and walking (includes at work and at home, walking to travel from place to place, and any other walking for recreation, sport, exercise, or leisure). The time spent on moderate and vigorous intensity activities was estimated in terms of frequency (days per week) and duration (minutes per day) taken in each of the two physical activity domains [
25]. Poor physical activity variable was computed as daily minutes (min/day) of physical activity scores in the work and walking domains. The variable was computed by summing up the time spent (min/day) in moderate-intensity and vigorous-intensity activities across the two domains such that if respondents took >10 minutes bouts of physical activity per day and <10 minutess bouts of physical activity per day they were considered physical active=0 and to have poor physical activity=1, respectively.Poor fruit and vegetables consumption was created when an individual reported daily consumption of less than the recommended 5 servings of fruit and vegetables. Respondents reported the number of servings for fruits/vegetables they had in a typical day, and if the servings were less than 5 in a day, they were considered to be having poor fruit/vegetable consumption [
26].
The NCD survey collected anthropometric information on height in meters (m) and weight in kilograms (kg) as per WHO guidelines [
27]. Body Mass Index (BMI) was used to classify overweight/obesity. BMI was derived from weight and height: weight (kg) / (height (m) x height (m)) [
1]. The Charder MS7301 250Kg digital scale and the Muac measuring tape were used for anthropometric measurements. Weight of respondents was measured, to the nearest 0.1 kg, while their height was measured in metres. BMI was categorized into: underweight (BMI < 18.5 kg/m
2), normal weight (18.5 ≤ BMI < 25 kg/m
2), overweight (25 ≤ BMI < 30 kg/m
2) and obese (BMI ≥30 kg/m
2) [
1]. Overweight and obese were used to create a binary outcome variable which was coded as: being overweight/obese (BMI ≥ 25 kg/m
2) =1; not overweight/obese =0 (BMI < 25 kg/m
2).
A wealth index (WI) was constructed and used as a measure of socioeconomic status (SES). It is a composite measure, constructed from the indicators of ownership of consumer durables, housing characteristics, and access to public services. Information on a wide range of durable assets was collected (e. g. ownership of telephone, car, refrigerator, television), housing characteristics (e. g. material of dwelling floor and roof, main cooking fuel), access to basic services (e. g. electricity supply, source of drinking water, sanitation facilities) and ownership of livestock (e.g. cattle, goats, sheep, horses, chickens). Further to the collection of information on durable assets, information on land and livestock ownership was collected. Principal component analysis was employed to derive the wealth index variable, which had five categories from the 1st to the 5th quintile (poorest to richest).
Control variables
Socio-demographic factors such as age, sex, marital status, work status, residence and education were used as controls for the analysis. These variables were conceptualized to have an association with NCD risk factors on the basis of the review of literature. Therefore to control for their likely association with NCD risk factors, they were included in the combined effects model, so that the association between the wealth status and NCD risk factors becomes isolated and discernible.
Statistical analysis
Logistic regression analysis was used to assess the association between socioeconomic variables and NCD risk factors using SPSS version 25. Results of logistic regression models were presented as adjusted odds ratios (AOR) together with their 95% confidence intervals.
Analysis of socioeconomic inequalities in NCD risk factors was done using ADePT software (version 6). The socioeconomic inequalities were derived using concentration curves and concentration indices. Concentration curves were used to plot the cumulative share of the NCD risk factor variables against the cumulative share of the wealth status variable. In calculating the cumulative percentages, wealth status was ranked from lowest to highest quintile. If any NCD risk factor was equally distributed, the curve would be running from the bottom left hand corner to the top right-hand corner (a 45° line). This is known as the line of equality. Contrarily, if the share of NCD risk factor was low among the poor, the concentration curve would lie below the line of equality [
28‐
30]. The further the curve is from the diagonal, the greater the degree of inequality. The first case of socioeconomic inequality is the case in which individuals with high SES have a positive value of concentration index., while the second case, where the curve is above the diagonal line, is known as socioeconomic inequality which disadvantages the individuals of lower SES and the value of the concentration index is negative [
31].
For the concentration index the value of the NCD risk factor assigned to each individual was taken to be a function of the socioeconomic category to which the individual belongs. The value of the concentration index ranges between − 1 to + 1. The index is 0 if there is no socioeconomic related inequality. The achievement index was also used with the concentration index to reflect the average level of NCD risk factors and the inequality in NCD risk factors between the poor and the better-off. It is the weighted average of the NCD risk factors of the various people in the sample, in which higher weights are attached to poorer people than to better-off people [
29]. The larger value of the index is considered as higher health disachievment to one group of population than others group. (Refer to Additional file
3 for the decomposition analysis equation).