title: Three challenges described for identifying participants with missing data in trials reports, and potential solutions suggested to systematic reviewers.
abstract: To categorize the challenges in determining the extent of missing participant data in randomized trials and suggest potential solutions for systematic review authors.
  
  
  
  During the process of updating a series of Cochrane systematic reviews on the topic of anticoagulation in patients with cancer, we identified challenges and used an iterative approach to improve, and a consensus process to agree on the challenges identified, and to suggest potential ways of dealing with them. The five systematic reviews included 58 trials and 75 meta-analyses for patient-important dichotomous outcomes with 27,037 randomized participants.
  
  
  
  We identified three categories of challenges: (1) Although systematic reviewers require information about missing data to be reported by outcome, trialists typically report the information by participant; (2) It is not always clear whether the trialists followed up participants in certain categories (e.g., noncompliers), that is, whether some categories of participants did or did not have missing data; (3) It is not always clear how the trialists dealt with missing data in their analysis (e.g., exclusion from the denominator vs. assumptions made for the numerator). We discuss potential solutions for each one of these challenges and suggest further research work.
  
  
  
  Current reporting of missing data is often not explicit and transparent, and although our potential solutions to problems of suboptimal reporting may be helpful, reliable and valid characterization of the extent and nature of missing data remains elusive. Reporting of missing data in trials needs further improvement.
authors.affiliation: Department of Internal Medicine, American University of Beirut, Riad-El-Solh Beirut 1107, Beirut 2020, Lebanon; Department of Clinical Epidemiology and Biostatistics, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4L8, Canada. Electronic address: ea32@aub.edu.lb.
authors.forename: Elie A
authors.initials: EA
authors.lastname: Akl
authors.affiliation: Department of Internal Medicine, American University of Beirut, Riad-El-Solh Beirut 1107, Beirut 2020, Lebanon.
authors.forename: Lara A
authors.initials: LA
authors.lastname: Kahale
authors.affiliation: Department of Clinical Epidemiology and Biostatistics, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4L8, Canada; Institute of Health Policy, Management and Evaluation, University of Toronto, 27 King's College Cir, Toronto, Ontario M5S, Canada; Department of Anesthesia and Pain Medicine, The Hospital for Sick Children, University Of Toronto, 27 King's College Cir, Toronto, Ontario M5S, Canada; Department of Medicine, Stanford University, 450 Serra Mall, Stanford, CA 94305, USA.
authors.forename: Shanil
authors.initials: S
authors.lastname: Ebrahim
authors.affiliation: Iberoamerican Cochrane Centre, Biomedical Research Institute Sant Pau (CIBERESP-IIB Sant Pau), Pavelló 18 Planta 0 Sant Antoni M Claret 167 08025, Barcelona, Spain.
authors.forename: Pablo
authors.initials: P
authors.lastname: Alonso-Coello
authors.affiliation: Department of Clinical Epidemiology and Biostatistics, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4L8, Canada; Department of Medicine, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4L8, Canada.
authors.forename: Holger J
authors.initials: HJ
authors.lastname: Schünemann
authors.affiliation: Department of Clinical Epidemiology and Biostatistics, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4L8, Canada; Department of Medicine, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4L8, Canada.
authors.forename: Gordon H
authors.initials: GH
authors.lastname: Guyatt
mesh_terms: D000925:Anticoagulants / Q000627:therapeutic use*; D035843:Biomedical Research / Q000379:methods*; D006801:Humans; D009369:Neoplasms / Q000188:drug therapy*; D016032:Randomized Controlled Trials as Topic; D012107:Research Design; D012196:Review Literature as Topic; D013223:Statistics as Topic / Q000379:methods*; D016896:Treatment Outcome
chemical_list: D000925:Anticoagulants
keywords: Attrition bias; Lost to follow-up; Missing participant data; Non-compliance; Randomized controlled trials; Systematic reviews