Letters to the Editor

Letters to the editor will be confined to discussions of papers that have appeared in The American Statistician and to important issues facing the statistical community. Letters discussing papers in The American Statistician must be received within two months of publication of the paper; the author of the paper will then be given an opportunity to reply, and the letters and reply will be published together. All letters to the editor will be refereed. Corrections of errors that have been noted in papers published in The American Statistician will be listed as corrections at the end of this section.     

Lemuel Shattuck,Statist Founder of the American Statistical Association

Polynomial regression of degree p in one independent variable χ is considered. Numerically large sample correlations between χ^α and χ^β, α < β, a, β = 1, ···, p, may cause ill-conditioning in the matrix to be inverted in application of the method of least squares. These sample correlations are investigated. It is confirmed that centering of the independent variable to have zero sample mean removes nonessential ill-conditioning. If the sample values of χ are placed symmetrically about their mean, the sample correlation between χ^α and χ^β is reduced to zero by centering when α + β is odd, but may remain large when α + β is even. Some examples and recommendations are given.