- BACKGROUND: Lack of appropriate reporting of methodological details has previously been shown to distort risk of bias assessments in randomized controlled trials. The same might be true for observational studies. The goal of this study was to compare the Newcastle-Ottawa Scale (NOS) assessment for risk of bias between reviewers and authors of cohort studies included in a published systematic review on risk factors for severe outcomes in patients infected with influenza. METHODS: Cohort studies included in the systematic review and published between 2008-2011 were included. The corresponding or first authors completed a survey covering all NOS items. Results were compared with the NOS assessment applied by reviewers of the systematic review. Inter-rater reliability was calculated using kappa (K) statistics. RESULTS: Authors of 65/182 (36%) studies completed the survey. The overall NOS score was significantly higher (p < 0.001) in the reviewers' assessment (median = 6; interquartile range [IQR] 6-6) compared with those by authors (median = 5, IQR 4-6). Inter-rater reliability by item ranged from slight (K = 0.15, 95% confidence interval [CI] = -0.19, 0.48) to poor (K = -0.06, 95% CI = -0.22, 0.10). Reliability for the overall score was poor (K = -0.004, 95% CI = -0.11, 0.11). CONCLUSIONS: Differences in assessment and low agreement between reviewers and authors suggest the need to contact authors for information not published in studies when applying the NOS in systematic reviews.