When meta-analyzing data from studies examining the incidence of rare events - or those with a small sample size or short follow-up period, it is not uncommon to come across a study with 0 events of the outcome of interest. In fact, approximately one-third of a random sample of 500 Cochrane reviews contained at least one zero-events study.
Zero-events studies are typically categorized as single-arm (there are 0 events reported in just one group) or double-arm (there are 0 events reported in both groups). While some software automatically discard double-arm zero-events studies from a meta-analysis, this is not ideal because these data still add useful information in regards to the overall effect of an intervention. Ideally, meta-analyses could include a pooled event count that may be zero in one arm, both arms, or neither, with various single-arm and double-arm zero-events studies potentially contributing to this final effect. Thus, in a recently published article, Xu and colleagues propose a more detailed framework for approaching zero-events studies in the context of a meta-analysis.
The authors describe six classifications as follows, with the degree of difficulty when meta-analyzing generally increasing from 1 to 6: