  This paper introduces a simple agglomerative clustering method to identify
large publishing consortia with at least 20 authors and 80% shared authorship
between articles. Based on Scopus journal articles 1996-2018, under these
criteria, nearly all (88%) of the large consortia published research with
citation impact above the world average, with the exceptions being mainly the
newer consortia for which average citation counts are unreliable. On average,
consortium research had almost double (1.95) the world average citation impact
on the log scale used (Mean Normalised Log Citation Score). At least partial
alphabetical author ordering was the norm in most consortia. The 250 largest
consortia were for nuclear physics and astronomy around expensive equipment,
and for predominantly health-related issues in genomics, medicine, public
health, microbiology and neuropsychology. For the health-related issues, except
for the first and last few authors, authorship seem to primary indicate
contributions to the shared project infrastructure necessary to gather the raw
data. It is impossible for research evaluators to identify the contributions of
individual authors in the huge alphabetical consortia of physics and astronomy,
and problematic for the middle and end authors of health-related consortia. For
small scale evaluations, authorship contribution statements could be used, when
available.
