The use of percentages and size-specific indices to normalize physiological data for variation in body size: wasted time, wasted effort?

Researchers commonly compute percentages or size-specific indices in an attempt to remove effects of body size from physiological data. Unfortunately, such ratios seldom eliminate the influence of body size on a physiological response and the ratios introduce major (but often unrecognized) problems with respect to statistical analysis and interpretation of the data. Indeed, these shortcomings of ratios frequently lead investigators to arrive at incorrect conclusions in otherwise flawless experiments. A superior alternative to using ratios combines graphical analysis and the analysis of covariance, which is a widely available statistical routine that uses least-squares regression to remove effects of body size from physiological data. Accordingly, we counsel researchers to discontinue forming ratios in an attempt to normalize physiological data for variation in body size and to adopt a reliable alternative. We also advise readers of scientific research not to place great confidence in results of studies that use ratios for scaling. (C) 1999 Elsevier Science Inc. All rights reserved.