Bridwell 1993.

Methods	Single‐centre RCT Setting: Barnes‐Jewish Hospital, St. Louis Missouri Country: USA Period: February 1985 to March 1990
Participants	Number: 44 patients (9/11/24) Diagnosis: magnetic resonance and computed tomographic imaging. Spinal claudication caused by spinal stenosis at the spondylolisthesis level Included: no previous spine surgery Excluded: not reported Age (years): mean (range) 66.1 (46‐79)
Interventions	Group 1: decompression alone. Surgical decompression comprised of laminectomy with preservation of bilateral facet joints without discectomy or extensive foraminotomy Group 2: decompression plus posterolateral (transverse processes) fusion without instrumentation or posterolateral (facets and transverse processes) fusion with instrumentation. All fusions were performed with autogenous iliac bone graft. Follow‐up: 37.2 months
Outcomes	Disability: Walking ability: worse, same or significantly better after surgery Complications Reoperations
Notes	Surgeon's experience: not reported Funding: Conflict of interest and financial support were not reported in this study.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	High risk	Quote: "the patients were randomized so that". The authors report an error in the randomisation process.
Allocation concealment (selection bias)	Unclear risk	Not mentioned.
Blinding of participants (performance bias) All outcomes	Unclear risk	No mention of any attempts to blind the participants.
Bliding of personnel/ care providers (performance bias)	High risk	The surgeon could not have been blinded to the intervention.
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	No mention of any attempts to blind the assessors.
Incomplete outcome data (attrition bias) All outcomes	Low risk	43/44=97.7% of the patients completed the follow‐up. The number of drop‐outs is unlikely to affect the results.
Intention‐to‐treat analysis (attrition bias)	Unclear risk	No information about intention‐to‐treat analysis.
Selective reporting (reporting bias)	High risk	Protocol not available, and relevant outcomes were not reported.
Group similarity at baseline (selection bias)	Unclear risk	No information about patient characteristics at baseline.
Co‐interventions (performance bias)	Unclear risk	Only the surgical technique differed between treatment groups. No concomitant discectomy, but foraminotomy was performed in some patients.
Compliance (performance bias)	Low risk	Compliance in both treatment groups: 100% (surgery).
Timing of outcome assessment (detection bias)	Low risk	All important outcome assessments for both groups were measured at the same time.
Other bias	Unclear risk	Conflict of interest not reported. Financial support was not reported in this study.