WoS covers about two-thirds of all submitted publications. Some fields are poorly covered in the WoS, such as the arts and humanities, having a coverage of only about $10$–$30\%$ of submitted publications, whereas the natural sciences generally have a high coverage of $90$–$95\%$ (see Table \[tab:estimate\_b\] for an overview). In the calculation of citation metrics, we take into account only publications covered in the WoS. In the calculation of statistics based on peer review, all publications submitted to the REF are considered, including those not covered in the WoS.

All matched publications are associated with a particular UoA, which roughly corresponds to a field or discipline. The REF 2014 distinguished $36$ UoAs. Every publication was submitted on behalf of a particular institution. Some publications were submitted in multiple UoAs, and we take them into account in each UoA. Publications that were co-authored and submitted by multiple institutions may thus be counted multiple times. Publications co-authored by several authors from the same institution were sometimes submitted multiple times in the same UoA by the same institution[^8]. We consider only the unique publications of an institution in a UoA. In other words, we count a publication only once, even if it was 