Always expect to find at least one error when you proofread your own statistics. If you don't, you are probably making the same mistake twice. Quote of american demographer Cheryl Russell appearing in "Rules of Thumb" by Tom Parker (Houghton Mifflin, 1983) p. 124. Also to be found in "Statistically Speaking the dictionary of quotations" compiled by Carl Gaither and Alma Cavazos-Gaither p. 81