Data collection

An initial invitation letter and a self-administered questionnaire including eight instruments were sent to the participants by the Swedish Social Insurance Agency. Several demographic variables such as age, cohabitation, number of children, education, years in the workforce, type of work, stress in the last 6 months, life-long pain duration, and physical activity were included in the questionnaire. In addition, a pain figure was included to collect information on the location of pain on the body [38]. Participants who agreed to take part in the study returned the questionnaire along with a signed informed consent form. To increase the response rate, two reminders were sent about two weeks apart. At the 1-year follow-up, the same questionnaire was sent to the baseline participants, this time including two additional background questions meant to detect RTW status.