The article did not provide actual numbers (which would certainly have helped!), and the sample was alarmingly small (only fifty people, total, with twenty-five each for the printed volume and the Kindle). Still, the outcome is disturbing, and further study appears warranted.
The text appears at http://www.washingtonpost.com/news/the-intersect/wp/2014/08/21/why-you-might-want-to-ditch-your-e-reader-and-go-back-to-printed-books/ .