Hurri 1989.

Methods	RCT
Participants	188 participants. 1. Back School group n = 95. 2. Instruction material of the Back School in written form group n = 93. Inclusion criteria: idiopathic LBP for at least 12 months, LBP present on at least 1 day each week during the preceding month, and/or ADL limitations. Exclusion criteria: rheumatoid arthritis or other connective tissue disease as well as people with a history of back surgery.
Interventions	1. Back School group: modified Swedish Back School: 60‐minute education and exercise sessions, 6 times in 3 weeks. Refresher course 2 x 60 minutes after 6 months. Supervised by physiotherapist; 11 participants per group. 2. Instruction material of the Back School in written form group: No actual treatment, but free to use healthcare services.
Outcomes	1. Pain: visual analogue scale. 2. Disability: Oswestry Disability Index.
Notes	Occupational setting. Funding: N/A.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	The participants were randomly assigned to one of two groups.
Allocation concealment (selection bias)	Unclear risk	The sequence generation procedure or the method of allocation were not mentioned. The title and abstract indicate that it is an RCT.
Blinding of participants and personnel (performance bias) All outcomes	High risk	No mention of any attempts to blind the participants
Blinding (performance bias and detection bias) All outcomes	High risk	No mention of any attempts to blind the care providers
Blinding of outcome assessment (detection bias) All outcomes	High risk	No mention of any attempts to blind the assessors
Incomplete outcome data (attrition bias) All outcomes	Low risk	The percentage of withdrawals and dropouts was within the acceptable rate (less than 20%).
Intention‐to‐treat Analysis	Unclear risk	Not mentioned
Selective reporting (reporting bias)	Unclear risk	It was unclear if the published report included all expected outcomes.
Similarity of baseline characteristics?	Low risk	The 2 groups were comparable for age and duration of low back pain syndrome.
Co‐interventions avoided or similar?	Low risk	Balanced for both groups
Compliance acceptable?	Low risk	Compliance was acceptable based on the reported intensity/dosage, duration, number, and frequency for both the intervention and control groups.
Timing outcome assessments similar?	Low risk	All important outcomes assessments for both groups were measured at the same time.