RE: When to do transformation of data?

From: "Kowalski, Ken" Subject: RE: [NMusers] When to do transformation of data? Date: Mon, 29 Apr 2002 16:16:15 -0400 Atul, To add further to Mats' comments regarding skewness in WRES, another diagnostic that might suggest the need for a log-transformation is when a concordance plot of PRED vs OBS shows bias but a plot of IPRED vs OBS does not. You can think of the log-transformation as providing a geometric mean prediction rather than an arithmetic mean prediction. The geometric mean is a better measure of central tendency when the data are right-skewed. A good discussion on this topic can be found in Carroll & Ruppert, Transformations and Weighting in Regression, Chapman & Hall, NY (1988). They devote a whole chapter to the "transform-both-sides" approach. Although the text is devoted to linear and nonlinear fixed effects models, many of the results are relevant for mixed effects models as well. Ken