Based on a previous study which reported a prevalence of low
back pain among rubber tappers of 55%, with a small design effect
due to systematic random sampling of 1.2 and an estimated nonresponse
rate of 10% that is usually used for field survey, the
required sample size was 502 subjects. This sample size is also
enough to test the hypothesis with a significance level of 0.05
which estimates that low back pain was 45% among an abnormal
ergonomic exposure group and 36% among a non-exposure group.
Due to no existing study of risk factors for low back pain among
rubber tappers, we estimated the prevalence of low back pain
among an abnormal ergonomic exposure group from a survey of
Thai agriculturists (Chooprapawan, 1996) who may have similar
ergonomic problems as rubber tappers and the prevalence among
non-exposure group from a survey in the general population
(Oksuz, 2006). This sample size was given 80% power for detecting
the differences of odds ratios between the ergonomic exposure and
non-exposure groups.