Wilson 2014.

Study characteristics
Methods	Retrospective cohort. Study period: 2005 to 2010
Participants	Country: USA. CD patients in National Surgical Quality Improvement Project database who underwent ileocolectomy
Interventions	1. Preoperative steroid therapy (n= 954) 2. No preoperative steroid therapy (n= 1515)
Outcomes	Organ space surgical site infection within 30 days of surgery
Notes	NOS low risk of bias overall
Risk of bias
Bias	Authors' judgement	Support for judgement
Representativeness of the exposed cohort	Low risk	CD patients in National Surgical Quality Improvement Project database who underwent ileocolectomy from 2005 to 2010
Selection of the non exposed cohort	Low risk	Both groups obtained from the same database and time period.
Ascertainment of exposure	Low risk	NSQIP database
Demonstration that outcome of interest was not present at start of study	High risk	Wound class classification IV in 12% of patients
Comparability of cohorts (Controlled for critical factor/other medications)	High risk	Did not control for other medications
Comparability of cohorts (Controlled for additional factor)	Low risk	Controlled for BMI, diabetes, smoking, ASA classification, extended operative time, open procedure, preoperative SIRS, emergent procedure, and wound classification
Assessment of outcome	Low risk	NSQIP database
Was follow‐up long enough for outcomes to occur	Low risk	30 days
Adequacy of follow up of cohorts	Unclear risk	Information not provided