The article states, "Research has shown that the ideal line length is about 65 characters." but it links to a pdf. I skimmed several pages and found one stateme

The article states, "Research has shown that the ideal line length is about 65 characters." but it links to a pdf. I skimmed several pages and found one statement that said what the average length was but I found mention of research on the ideal line length. The Smashing Magazine article linked says 65 is just a general rule derived from 2.5 times the Roman Alphabet. Is there any actual research on this number?