Assessments of attrition bias in Cochrane systematic reviews are highly inconsistent and thus hindering trial comparability

Table 8 Examples of vague instructions for Cochrane authors regarding judgments of attrition bias

Quote from a Cochrane review	Comment of authors of this study
Missing outcome data balanced in numbers across intervention groups, with similar reasons for missing data across groups	There is no quantitative measure of “balanced”
For dichotomous outcome data, the proportion of missing outcomes compared with observed event risk not enough to have a clinically relevant impact on the intervention effect estimate	There is no quantitative measure of “not enough”
Missing data have been imputed using appropriate methods	Not specified what is considered by Cochrane to be “appropriate methods”
Reason for missing outcome data likely to be related to true outcome, with either imbalance in numbers or reasons for missing data across intervention groups	There is no quantitative measure of “imbalance”
Potentially inappropriate application of simple imputation.	Not specified what is considered to be “inappropriate application”