*2.2. Settings*

This study was conducted at a hospital in the town of Obuse from October 2014 to June 2017.

## *2.3. Bias*

In order to minimize selection bias, we randomly selected candidates from the basic town resident registry.

#### *2.4. Study Size*

Assuming that the frequency of back pain in the comparison group was between 5% and 20%, sample size calculation estimated that 89 subjects per group would provide 80% statistical power (1 minus beta) with an alpha equal to 0.05. After estimating the possible cohort size in consideration of budget, time, and burden on subjects and research staff, we planned to establish eight groups by age (50's, 60's, 70's, and 80's) and gender (male and female) containing approximately 50 subjects each for a total of at least 400 subjects.

#### *2.5. Data Source*

The subject selection process in this study has been previously reported [10]. Briefly, we randomly sampled for candidates from the basic resident registry of the Obuse town (population: 11,326 in 2014). Sampling was conducted until the number of individuals providing consent for study participation reached the target number. A total of 1297 individuals were randomly selected from 5352 people aged between 50 and 89 years in the basic resident registry of the Obuse town in 2014 (Figure 1) [10]. Of them, 882 people were unwilling to participate for undisclosed reasons and excluded from this study.
