Kowalski 2016.

Study characteristics
Methods	RCT
Participants	1678 people having elective general surgery in the USA between October 2009 and February 2015 who were considered to have "sufficient androgenic hair"
Interventions	Group 1: hair removed with clippers (n = 834) Group 2: no hair removed (n = 844) Product details: clippers were single‐use disposable electric clippers. Timing of hair removal: immediately before surgery. Hair removed by: not specified. Venue for hair removal: not specified.
Outcomes	Primary outcome: SSI defined by the CDC criteria. SSI was assessed by an independent research nurse at the participant's follow‐up visit. Research nurses also attempted to contact participants at 30 days postoperatively. Secondary outcomes: classification of SSI using demographic, clinical, and surgical data collected by study nurses. Surgeons were asked to comment, via a questionnaire, on the impact of hair removal on ease of surgery.
Notes	Disclosure: authors have nothing to disclose. Portions of the study were funded by Gundersen Health Systems and the Gundersen Medical Foundation.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Block design with a block size of 100. Comment: consider adequate approach.
Allocation concealment (selection bias)	Low risk	Sealed envelopes. Comment: consider adequate approach.
Blinding (performance bias and detection bias) Care providers blinded	Unclear risk	Not reported. Comment: unclear whether care providers were blinded to intervention allocation.
Blinding (performance bias and detection bias) Participants blinded	High risk	Not reported. Comment: participants would have been aware of hair removal method.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: SSI was assessed by an independent nurse not directly involved with the patients' care. Comment: consider adequate approach.
Incomplete outcome data (attrition bias) ITT analysis undertaken	Low risk	ITT analysis was conducted. Comment: consider adequate approach.
Incomplete outcome data (attrition bias) Drop out rate acceptable	Low risk	Participants who dropped out are accounted for, and participants were distributed evenly across groups. Comment: the number of dropouts was judged to be unlikely to have altered the result, even in a worst‐case scenario (i.e. assuming that those that dropped out developed an SSI).
Selective reporting (reporting bias)	Low risk	Complied with protocol, supported by funding but not from a pharmaceutical company. Comment: unlikely to be affected by reporting bias.
Other bias	Low risk	Comment: participant groups were equal or similar.