Imputation methods for missing outcome data in meta-analysis of clinical trials

Background Missing outcome data from randomized trials lead to greater uncertainty and possible bias in estimating the effect of an experimental treatment. An intention-to-treat analysis should take account of all randomized participants even if they have missing observations. Purpose To review and develop imputation methods for missing outcome data in meta-analysis of clinical trials with binary outcomes. Methods We review some common strategies, such as simple imputation of positive or negative outcomes, and develop a general approach involving 'informative missingness odds ratios' (IMORs'). We describe several choices for weighting studies in the meta-analysis, and illustrate methods using a meta-analysis of trials of haloperidol for schizophrenia. Results IMORs describe the relationship between the unknown risk among missing participants and the known risk among observed participants. They are allowed to differ between treatment groups and across trials. Application of IMORS and other methods to the haloperidol trials reveals the overall conclusion to be robust to different assumptions about the missing data. Limitations The methods are based on summary data from each trial (number of observed positive outcomes, number of observed negative outcomes and number of missing outcomes) for each intervention group. This limits the options for analysis, and greater flexibility would be available with individual participant data. Conclusions We propose that available reasons for missingness be used to determine appropriate IMORs. We also recommend a strategy for undertaking sensitivity analyses, in which the IMORs are varied over plausible ranges.