In 1997, low back pain was described as “chronic spinal affection for longer than 3 months”, while in 2001 and 2004 “lumbago, sciatica, and disc prolapse” was added to the 1997 question. Nonetheless, in 2008, the question changed to “low back disorder or other chronic back defect”. The impact of the changes in the questions can be observed in the disease prevalence by gender and BHIS year, shown in the Additional file