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Analyzing and Presenting pH Data

Department of Dairy Science, University of Illinois, Urbana 61801

ABSTRACT

Both the practical effects and theoritical basis of error in pH data analysis and presentation were examined. Since the "pH" transformation has been applied for convenience and not for statistical reason, added care must be exercised in analyzing these data. Calculating a mean of observed pH's introduced a significant and predictable bias as compared to –log (mean [H⁺]), henceforth designated mH. Rumen pH data from Holstein steers fed a high concentrate diet twice daily showed that a relative error of from .35 to 2.22%, with an average of 1.25%, resulted when the former method was used. Wide ranges in observed pH or low pH greatly increased the possible relative error. Statistical analyses of pH data should use the untransformed variable, i.e., hydrogen ion concentration. Mean hydrogen ion concentration, the variable in which researchers are interested, still can be transformed for convenience and reported as mH together with the asymmetric confidence intervals that result from transformation.

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