allow for single-arm studies (proportions/prevalences/...)

[christianroever]

Data examples:

metadat::dat.crisafulli2020 (26 studies)

Neuenschwander & al. (2010) (four studies).
https://doi.org/10.1002/jrsm.1475 (Sec.5.3.3, Tab.11 and Fig.6).