A 2010 study identified a correlation between higher student grades and courses that used open textbooks. While there remain (some understandable) issues around isolating the particular influence of openness they found that students in courses that used open textbooks tended to have higher grades and lower failing and withdrawal rates than those in courses that did not use open texts.
This study reports findings from a year-long pilot study during which 991 students in 9 core courses in the Virginia State University School of Business replaced traditional textbooks with openly licensed books and other digital content. The university made a deliberate decision to use open textbooks that were copyrighted under the Creative Commons license. This decision was based on the accessibility and flexibility in the delivery of course content provided by open textbooks. More students accessed digital open textbooks than had previously purchased hard copies of textbooks. Higher grades were correlated with courses that used open textbooks.