Life Science’s Average Publishable Unit (APU) Has Increased over the Past Two Decades

<div><p>Quantitative analysis of the scientific literature is important for evaluating the evolution and state of science. To study how the density of biological literature has changed over the past two decades we visually inspected 1464 research articles related only to the biological sciences from ten scholarly journals (with average Impact Factors, IF, ranging from 3.8 to 32.1). By scoring the number of data items (tables and figures), density of composite figures (labeled panels per figure or PPF), as well as the number of authors, pages and references per research publication we calculated an Average Publishable Unit or APU for 1993, 2003, and 2013. The data show an overall increase in the average ± SD number of data items from 1993 to 2013 of approximately 7±3 to 14±11 and PPF ratio of 2±1 to 4±2 per article, suggesting that the APU has doubled in size over the past two decades. As expected, the increase in data items per article is mainly in the form of supplemental material, constituting 0 to 80% of the data items per publication in 2013, depending on the journal. The changes in the average number of pages (approx. 8±3 to 10±3), references (approx. 44±18 to 56±24) and authors (approx. 5±3 to 8±9) per article are also presented and discussed. The average number of data items, figure density and authors per publication are correlated with the journal’s average IF. The increasing APU size over time is important when considering the value of research articles for life scientists and publishers, as well as, the implications of these increasing trends in the mechanisms and economics of scientific communication.</p></div

Change in the average first-quartile publishable unit (AQ1PU) over the past two decades.

<p>Box plots showing the change in the lower quartile group (25^th percentile and less) of the APU as shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0156983#pone.0156983.g001" target="_blank">Fig 1</a>.</p

Increase in the Average Publishable Unit (APU) over the past two decades of scientific publication.

<p>A total of 1464 research articles were visually-inspected to quantify: the average number of (<b>a</b>) total data items (including supplemental material, +SM); (<b>b</b>) data items in main article alone (-SM); (<b>c</b>) PPF ratio or panels inside composite figures (-SM); (<b>d)</b> pages (-SM), (<b>e</b>) references (+SM), and (<b>f</b>) authors per publications in 1993 (white), 2003 (blue), and 2013 (red) by 10 scholarly journals with diverse average IF (in parenthesis). Box plots display the average median (horizontal line in the box interior), interquartile range or distance between the 25^th and 75^th percentiles (box limits), and the minimum and maximum values (vertical lines issuing from the box) for each variable measured. The mean is indicated by the “+” symbol inside the box interior. Averages considering all articles (487 articles for 1993, 484 for 2003, and 493 for 2013) from all journals are shown on the right-end (note divisor line). Kruskal-Wallis non-parametric statistical test: *P≤0.05; **P≤0.01; ***P≤0.001; ****P≤0.0001. For the year 2013, top and bottom asterisks correspond to statistical analysis between 2013–1993 and 2013–2003, respectively.</p

Correlation between the APU variables and journal’s average IF.

<p>Linear regression and Spearman’s rank-order correlation (Spearman’s critical rho (N = 10, alpha = 0.05) = 0.648) analysis of average IF and average number of (<b>a</b>) total data items (including supplemental material, +SM) (R² = 0.14, p = 0.04; Spearman’s rho = 0.65, p = 0.05): (<b>b</b>) data items in main article alone (-SM) (R² = 0.17, p = 0.03; rho = -0.56, p = 0.10); (<b>c</b>) PPF (R² = 0.40, p = 0.0002; rho = 0.96, p<0.0001); (<b>d</b>) pages (R² = 0.01, p = 0.71; rho = -0.08, p = 0.84); (<b>e</b>) references (R² = 0.10, p = 0.08; rho = 0.50, p = 0.14); and (<b>f</b>) authors (R² = 0.44, p<0.0001; rho = 0.81, p = 0.007) per publication. Error bars represent standard deviations.</p