title: Blinding in randomized controlled trials in general and abdominal surgery: protocol for a systematic review and empirical study.
abstract: BACKGROUND
  Blinding is a measure in randomized controlled trials (RCT) to reduce detection and performance bias. There is evidence that lack of blinding leads to overestimated treatment effects. Because of the physical component of interventions, blinding is not easily applicable in surgical trials. This is a protocol for a systematic review and empirical study about actual impact on outcomes and future potential of blinding in general and abdominal surgery RCT.
  
  
  METHODS
  A systematic literature search in CENTRAL, MEDLINE and Web of Science will be conducted to locate RCT between 1996 and 2015 with a surgical intervention. General study characteristics and information on blinding methods will be extracted. The risk of performance and detection bias will be rated as low, unclear or high according to the Cochrane Collaboration's tool for assessing risk of bias. The main outcome of interest will be the association of a high risk of performance or detection bias with significant trial results and will be tested at a level of significance of 5 %. Further, trials will be meta-analysed in a Mantel-Haenszel model comparing trials with high risk of bias to other trials at a level of significance of 5 %.
  
  
  CONCLUSIONS
  Detection and performance bias distort treatment effects. The degree of such bias in general and abdominal surgery is unknown. Evidence on influence of missing blinding would improve critical appraisal and conduct of general and abdominal surgery RCT.
  
  
  BACKGROUND
  PROSPERO 2015: CRD42015026837.
authors.affiliation: Department of General, Visceral and Transplantation Surgery, University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.|The Study Center of the German Surgical Society (SDGC), University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.
authors.forename: Pascal
authors.identifier: http://orcid.org/0000-0002-0895-4015
authors.initials: P
authors.lastname: Probst
authors.affiliation: The Study Center of the German Surgical Society (SDGC), University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.
authors.forename: Kathrin
authors.initials: K
authors.lastname: Grummich
authors.affiliation: Department of General, Visceral and Transplantation Surgery, University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.|The Study Center of the German Surgical Society (SDGC), University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.
authors.forename: Patrick
authors.initials: P
authors.lastname: Heger
authors.affiliation: Department of General, Visceral and Transplantation Surgery, University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.|The Study Center of the German Surgical Society (SDGC), University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.
authors.forename: Steffen
authors.initials: S
authors.lastname: Zaschke
authors.affiliation: Department of General, Visceral and Transplantation Surgery, University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.|The Study Center of the German Surgical Society (SDGC), University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.
authors.forename: Phillip
authors.initials: P
authors.lastname: Knebel
authors.affiliation: Department of General, Visceral and Transplantation Surgery, University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.
authors.forename: Alexis
authors.initials: A
authors.lastname: Ulrich
authors.affiliation: Department of General, Visceral and Transplantation Surgery, University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany.
authors.forename: Markus W
authors.initials: MW
authors.lastname: Büchler
authors.affiliation: Department of General, Visceral and Transplantation Surgery, University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany. markus.diener@med.uni-heidelberg.de.|The Study Center of the German Surgical Society (SDGC), University of Heidelberg, Im Neuenheimer Feld 110, 69120, Heidelberg, Germany. markus.diener@med.uni-heidelberg.de.
authors.forename: Markus K
authors.initials: MK
authors.lastname: Diener
mesh_terms: D000005:Abdomen / Q000601:surgery*; D015982:Bias; D004311:Double-Blind Method; D006801:Humans; D016032:Randomized Controlled Trials as Topic / Q000379:methods*; D016037:Single-Blind Method; D000078202:Systematic Reviews as Topic
keywords: Bias; Blinding; Detection bias; General and abdominal surgery; Performance bias; Randomized controlled trial; Risk of bias; Systematic review