Data on back pain (defined as pain in the neck, mid back and/or lower back) were collected once a
week from parents replying to automated text-messages over 2.5 school years from 2008 till 2011. The prevalence
estimates were presented as percentages and 95% confidence intervals. Differences between estimates were
considered significant if confidence intervals did not overlap. A test for trend, using a multi-level mixed-effects
logistic regression extended to the longitudinal and multilevel setting, was performed to see whether back pain
reporting increased with age.