The Necessity of Independent Testing of Soil‐Site Equations

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Abstract
A black oak (Quercus velutina) soil‐site study in Missouri has shown that the standard regression statistics (R2, standard error, etc.) of soil‐site equations are not reliable indicators of the accuracy of these equations. Data from 54 plots were used to derive a soil‐site equation that had an R2 of 0.66 and a standard error of 1.00 m. These statistics plus a graphed comparison of observed vs. predicted site indexes indicated that the equation satisfactorily predicted site index. When the equation was tested with data from 27 additional plots, however, the correlation between the observed and predicted site indexes had an R2 of only 0.01 and a standard error of 1.81 m. These statistics plus a graphed comparison of observed vs. predicted site indexes showed that the equation was completely unreliable as a site index predictor.Inadequate sampling, incomplete data analysis, or chance correlations may have caused the errors in the equation, but the errors were undetectable until the equation was tested with the independent data.

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