Table 1 Reasons for omission, adapted from Yordanov et al. [10]

Inadequate planning

Whether the outcome was not planned according to the protocol nor reported in the trial reports

Selective reporting

Whether the outcome was planned according to the protocol, but not reported in the trial reports

Incomplete reporting

Whether the outcome was reported in the trial reports, but not in a way that allowed its inclusion in the meta-analysis (i.e., medians and interquartile ranges with no reference to the distribution of the data, data presented as means with no measure of variability, data presented graphically only, results presented just as p-values, Z or t values)

Unable to distinguish between selective reporting and inadequate planning

Whether no protocol or registry entry was available and the outcome was not reported in the reports

Other situations

The dichotomous outcome was listed in the trial reports, but there was no event

The outcome concerned adverse events, but there was no event

If it was judged that the outcome had been reported in a way that would allow it to be included in the meta-analysis, but the SR authors did not include it

Justified to be not included

In case the study could not be included in the meta-analysis for reasons related to the nature of the data (e.g., results reported as median and interquartile range as they were not normally distributed, or reported as mean change and standard deviation or 95% confidence interval) or to specific choices of the SR authors (e.g., the SR only included in the meta-analysis studies that had assessed an outcome with one or more selected outcome measures and the study in question assessed the same outcome with a different outcome measure)

Not assessed—Language

If the study was published in a language other than English

Not assessed—Not found and not possible to judge

In case the full text of the study was not retrieved