Outcomes, definitions and statistical methods
| Definition | Statistical method | |
| Primary outcome | ||
| Number of recurrences of spinal pain (3–27 months follow-up). | (1) A positive answer on the weekly text message for spinal pain (2) minimum of 1 week without report of spinal pain prior to the recurrence. | A hierarchical negative binomial regression model was used. Intervention effects were expressed as incidence rate ratio. |
| Secondary outcomes | ||
| Average duration of spinal pain episodes. | The number of consecutive weeks the child was affected by spinal pain (response option ‘1’). | A mixed-effects linear regression model with subject as random effect, outcome log transformed was used. Intervention effects were expressed as the difference in median length. |
| Total duration of complaint time in relation to individual follow-up time. | Total number of weeks a child was affected by spinal pain (response option ‘1’) in the entire follow-up period. | A hierarchical negative binomial regression model was used. Intervention effects were expressed as incidence rate ratio. |
| Global Perceived Effect after 2 weeks. | Dichotomised into two groups: ‘Much better’ and ‘The same or worse’. | A logistic regression model was used. Intervention effects were expressed as ORs. |
| Change in pain intensity after 2 weeks. | Rated on an 11-point Numerical Rating Scale with ‘0’ being ‘no pain’ and ‘10’ being ‘worst pain’. | A linear regression model was used. Intervention effects were expressed as the difference in mean length. |