Larger Optic Nerve Heads Have More Nerve Fibers in Normal Monkey Eyes

To the Editor. —We would like to draw attention to the statistical interpretation of data represented by Quigley et al¹in the October 1991 issue of theArchives. In Fig 1, the relationship between optic nerve fibers and optic disc is plotted with a linear regression correlation coefficient (r=.47,P<.05). On examination of the plot, these data seem to have a poor linear fit and, in fact, thervalue of.47 confirms this observation. Ifr=1, there is a perfect linear relationship and all the points lie on a straight line. Ifr=0, there is no linear relationship. Even though the correlation coefficient is greater than zero and is significant, this does not imply a good linear relationship. The real strength of a linear relationship is best indicated byR², which is a measure of the proportion of variation accounted for in the linear regression.