"At one point they set cell L51 equal to AVERAGE(L30:L44) when the correct procuedure was AVERAGE(L30:L49). By typing wrong, they accidentally left Denmark, Canada, Belgium, Austria, and Australia out of the average. When you run the math correctly "the average real GDP growth rate for countries carrying a public debt-to-GDP ratio of over 90 percent is actually 2.2 percent, not -0.1 percent."
http://www.peri.umass.edu/236/hash/31e2ff374b6377b2ddec04deaa6388b1/publication/566/