Number of references: a large-scale study of interval ratios

The paper presents a large-scale study (covering 26,998,764 items) of the development in the number of references over time (1996–2019) in three document types (articles; reviews; notes) from seven fields (Arts and Humanities; Social Sciences; Computer Science; Mathematics; Engineering; Medicine; Physics and Astronomy). Using interval ratios instead of average numbers, the paper makes it possible to follow the development, and to locate the main causes of growth in the number of references over time. The results show significant differences between fields and document types. The number of references in journal articles and reviews are growing in all fields (except for the reviews in Arts and Humanities that remain stable over time), but at different pace; The number of references in notes are growing in some fields (again at different pace) and are stable in others. The observed growth is primarily caused by a drop in short reference lists and a corresponding increase in a bit longer and medium size reference lists. Long and very long reference lists remain much more stable in shares over time, and does therefore not contribute much to the observed growth. The results underline the importance of normalizing citation data, and for taking citation inflation into account when conducting citation analyses expanding different fields, document types, and longer time-periods.