Background: Direct empirical evidence for the existence of outcome reporting bias is accumulating and this source of bias is recognised as a potential threat to the validity of metaanalysis of randomised clinical trials. Methods: A method for calculating the maximum bias in a meta-analysis due to publication bias is adapted for the setting where within-study selective non-reporting of outcomes is suspected, and compared to the alternative approach of missing data imputation. The properties of both methods are investigated in realistic small sample situations. Results: The results suggest that the adapted Copas and Jackson approach is the preferred method for reviewers to apply as an initial assessment of robustness to within-study selective nonreporting. Conclusion: The Copas and Jackson approach is a useful method for systematic reviewers to apply to assess robustness to outcome reporting bias.