Heterogeneity will be greater in a systematic review of NRS than in a systematic review than of randomized trials. Therefore, authors should consider very carefully the likely extent of heterogeneity between included studies when deciding whether to pool findings quantitatively (i.e. by meta-analysis). We expect pooling of effect estimates from NRS to be the exception, rather than the rule;
Effect estimates from NRS should not be combined with effect estimates from randomized trials, or with NRS that dissimilar study design features;
Forest plots should be used to summarize the findings from included studies;
Heterogeneity diagnostics and investigations may be used irrespective of whether or not a decision has been taken to pool effect estimates from different studies.