Axis limits are set to include at least 99% of the data. ( E) Relationship between FRE and NDC scores across all abstracts, depicted by a two-dimensional kernel density estimate. Color scales are linear and represent relative density of scores within each year. ( C,D) Kernel density estimates displaying the readability (C: FRE, D: NDC) distribution of all abstracts for each year. ( B) Mean New Dale-Chall (NDC) readability for each year. ( A) Mean Flesch Reading Ease (FRE) readability for each year. Scientific abstracts have become harder to read over time. Lower readability is indicated by a low FRE score or a high NDC score ( Figure 1A). Difficult words are defined as those words which do not belong to a predefined list of common words (see MaterialsВ andВ methods). The NDC is calculated using the number of words in each sentence and the percentage of 'difficult words'. The FRE is calculated using the number of syllables per word and the number of words in each sentence. We quantified the reading level of each abstract using two established measures of readability: the Flesch Reading Ease (FRE Flesch, 1948 Kincaid et al., 1975)В and the New Dale-Chall Readability Formula (NDC Chall and Dale, 1995). This journal list included, among others, Nature, Science, NEJM, The Lancet, PNAS and JAMA (see MaterialsВ andВ methods and Supplementary file 1) and the publication dates ranged from 1881 to 2015. These journals cover general, biomedical and life sciences. To investigate trends in scientific readability over time, we downloaded 709,577 article abstracts from PubMed, from 123 highly cited journals selected from 12 fields of research ( Figure 1AвЂ“C). A tentative increase in complexity has been reported in scientific texts in a limited dataset ( Hayes, 1992), but the extent of this phenomenon and any underlying reasons for such a trend remain unknown.

One study showed an increase in positive sentiment ( Vinkers et al., 2015), finding that positive words such as 'novel' have increased dramatically in scientific texts since the 1970s. There are studies that have investigated linguistic trends within the scientific literature. Texts written at different times can vary in their readability: trends towards simpler language have been observed in US presidential speeches ( Lim, 2008), novels ( Danielson et al., 1992 Jatowt and Tanaka, 2012) and news articles ( Stevenson, 1964). The clarity of written language can be quantified using readability formulas, which estimate how understandable written texts are ( Flesch, 1948 Kincaid et al., 1975 Chall and Dale, 1995 Danielson, 1987 DuBay, 2004 Е tajner et al., 2012). Reporting science clearly and accurately is a fundamental part of the scientific process, facilitating both the dissemination of knowledge and the reproducibility of results.

Flesch readability scale book