"The Inefficient Stock Market - What Pays Off And Why". Book by Robert Haugen, ch. 3, Estimating Portfolio Risk and Expected Return with Ad Hoc Factor Models, p. 29, 1999.
"Author Ernie Cline Was Told Video Games Would Rot His Brain, But Who Holds the High Score Now?". Interview with Michael Mechanic, www.motherjones.com. July 10, 2015.