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Identifying the ‘incredible’! Part 1: assessing the risk of bias in outcomes included in systematic reviews
  1. Fionn Büttner1,
  2. Marinus Winters2,
  3. Eamonn Delahunt1,3,
  4. Roy Elbers4,
  5. Carolina B Lura2,
  6. Karim M Khan5,
  7. Adam Weir6,7,8,
  8. Clare L Ardern9,10,11
  1. 1 School of Public Health, Physiotherapy and Sports Science, University College Dublin, Dublin, Ireland
  2. 2 Center for General Practice at Aalborg University, Aalborg, Denmark
  3. 3 Institute for Sport & Health, University College Dublin, Dublin, Ireland
  4. 4 Population Health Sciences, Bristol Medical School, University of Bristol, Bristol, UK
  5. 5 Department of Family Practice, The University of British Columbia, Vancouver, British Columbia, Canada
  6. 6 Sports Groin Pain Centre, Aspetar Orthopaedic and Sports Medicine Hospital, Doha, Qatar
  7. 7 Department of Orthopaedics, Erasmus MC University Medical Center for Groin Injuries, Rotterdam, The Netherlands
  8. 8 Sport Medicine and Exercise Clinic Haarlem (SBK), Haarlem, The Netherlands
  9. 9 Sport and Exercise Medicine Research Centre, La Trobe University, Bundoora, Victoria, Australia
  10. 10 Department of Medicine and Health Sciences, Division of Physiotherapy, Linköping University, Linköping, Sweden
  11. 11 Division of Physiotherapy, Karolinska Institute, Stockholm, Sweden
  1. Correspondence to Fionn Büttner, School of Public Health, Physiotherapy and Sports Science, University College Dublin, Dublin 4, Ireland; fionn.cleirigh-buttner{at}

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Systematic reviews fulfil a vital role in modern medicine.1 However, the results of systematic reviews are only as valid as the studies they include.2 Pooling flawed, or biased, results from different studies can compromise the credibility of systematic review findings. Bias is a systematic deviation from the truth in the results of a research study that can manifest due to limitations in study design, conduct, or analysis.3

The results of sport and exercise medicine research, like results in other fields, are vulnerable to bias.4 It is important that systematic review authors assess for bias in a way that enables a judgement about whether a review outcome is at risk of bias due to methodological limitations in included studies. This two-part education primer focuses on how systematic review authors can perform and interpret risk of bias assessments to avoid misleading systematic review conclusions. In this editorial, we introduce the concept of risk of bias, and the principles of assessing risk of bias.

Bias: the basics

Different biases have effects that vary in direction and magnitude.3 5 It is challenging to precisely determine how bias may overestimate or underestimate a study’s true findings. In fact, bias does not always result in distorted study findings and one can never be certain that bias is present when a study has methodological limitations. However, methodological limitations in study design, conduct, or analysis can be consistently associated with inflated research findings.5 Due to this uncertainty, study outcomes are considered to be at risk of bias rather than ‘biased’.

Studies with ‘some concerns’ or at ‘high’ risk of bias in design, conduct, analysis, or reporting are at greater risk of inflated findings compared with studies at ‘low’ risk of bias, negatively …

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  • Twitter @peanutbuttner, @marinuswinters, @EamonnDelahunt, @clare_ardern

  • Contributors MW and CLA conceived the original idea. FCB, MW, ED, and CLA developed the original idea. FCB composed the initial manuscript draft. MW, ED, and CLA provided comments on and contributed towards the writing of the initial manuscript draft. FCB, MW, ED, RE, CBL, KMK, AW, and CLA provided comments on and contributed towards the writing and editing of the final manuscript draft.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests Karim M Khan is BJSM Editor-in-Chief. Adam Weir is a BJSM Deputy Editor. Eamonn Delahunt and Marinus Winters are BJSM Senior Associate Editors. Clare L Ardern was a BJSM Deputy Editor until July 2018. Roy Elbers is a member of the ROB2 Development Group.

  • Patient consent for publication Not required.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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