A 2015 study investigated the frequency at which difficult vocabulary from the WORDSUM test were employed in about 5.9 million English-language texts published between 1850 and 2005. The researchers found that the more difficult of words were in declining usage and a negative correlation between the use of such words and completed fertility. On the other hand, simpler words entered increasingly common use, an effect of rising literacy. In another study, from 2017, researchers employed Google's Ngram Viewer, an enormous archive of scanned books, periodicals, and other printed materials dating back to the sixteenth century. They found that the use of difficult vocabulary increased substantially between the mid-1700s and mid-1800s before declining steadily till the present day.