Letters to the editor in exercise science and physical therapy journals: an examination of content and “authorship inflation”

Letters to the editor are often critiques of published research papers. Journal editors acknowledge the importance of letters in post-publication review, yet the themes of letters remain unclear. Incidentally, letters can also be used to examine “authorship inflation” in academic publishing, as confounders (e.g., research complexity) are naturally controlled by the letter model. Thus, the aim of Study 1 was to describe the contents of letters published in nine exercise science and physical therapy journals between 2000 and 2018 (n = 1047 letters). The aim of Study 2 was to determine if mean number of authors per letter increased between 1963 and 2018 in seven exercise science and physical therapy journals (n = 2247 letters). Letter writers most commonly commented on results interpretation (52.7% of letters), methods (52.5%), referencing (12.1%), statistics (11.7%), terminology or definitions (5.0%), data errors (4.1%), and ethical or safety issues (2.5%). Letter writers also frequently referenced their own work (51.8%) and provided compliments (31.5%). In Study 2, when data from all journals were combined and categorized in 5-year epochs, mean number of authors per letter generally increased over time: 1980–84 (1.30 ± 0.66; mean ± SD), 1985–89 (1.32 ± 0.64), 1990–94 (1.56 ± 1.42), 1995–99 (1.47 ± 0.87), 2000–04 (1.55 ± 0.95), 2005–09 (1.80 ± 1.11), 2010–14 (2.11 ± 1.27), and 2015–18 (2.73 ± 3.21). Percentage of single-author letters decreased from 77.9% in 1980–84 to 30.6% in 2015–18. Overall, the results clarify the role of letters in post-publication review. They also suggest “authorship inflation” in letters published in scientific journals.