tldr: neither journal appears to be pursuing a model of selectivity
PLOS Biology and PLOS Medicine are called “selective” journals by the publisher and researchers in the scientific community. Nevertheless, both journals underwent substantial growth in recent years, while experiencing decreasing Journal Impact Factors and, for PLOSBiology, a monotonic decline in its Article Influence Score.
As measured by the number of Research Articles published (data: PLOS), PLOS Biology grew by 51% from 2017 to 2018, and by 38% from 2018 to 2019 (Figure 1). Unless the journal has stockpiled a number of unpublished papers that will be published in November and December, total output will likely shrink substantially in 2020. This change in output appears to reflect a recent change in the scope of the journal, which now focuses more on process (preregistration and study design) and less on results.
While no structural change has been made at PLOS Medicine, the journal also experienced a large increase in published papers, from 170 in 2018 to 230 in 2019, or an increase of 35%. In the first ten months (Jan-Oct) of 2020, PLOS Medicine has already surpassed its 2019 output (Figure 2). At current output levels, PLOS Medicine is expected to publish a total of 331 papers in 2020.
Both journals have experienced downward trends in their citation performances. PLOS Biology received its maximum Journal Impact Factor (JIF) of 14.433 in 2005; its latest (2019) JIF was 6.909 (Figure 1). PLOS Medicine received its maximum JIF of 15.736 in 2011; its 2019 JIF was 10.298. An increase in article output in 2019 (especially in the last quarter of the year for Medicine, Figure 2) will likely depress both journal’s 2020 JIFs. We expect PLOSBiology’s next JIF to drop below 7, placing the journal below BMC Biology. We expect PLOSMedicine’s next JIF to fall below 10, placing it among topical and regional medical journals.
As measured by Article Influence Score (AIS), a metric that weights citations based on the source, the AIS for PLOS Medicine has remained relatively constant from year to year, while the AIS for PLOS Biology has undergone a sustained monotonic decline (Figure 1).
The rapid rise in publication output in 2019 indicates that neither journal is pursuing a model of selectivity. Indeed, PLOS Biology has publicly defined a new course that will focus on process and not on scientific importance or novelty. Moreover, citation performances of both journals will suffer next year, making it more difficult to attract important papers to either title. As the publisher is currently pursuing a model away from Article Processing Charges (APC) for these journals and toward a shared community model, such performance declines may send a contradictory message to scientific authors and make it more difficult to attract subscribing institutions.
 As the JIF is based on the performance of papers published in the previous two years, journal growth results in a larger number of younger (lower performing) papers being evaluated. Conversely, a decrease in output tends to inflate the JIF as there are more older (higher performing) papers being evaluated. The JIF is published annually in June by Clarivate Analytics.