Model Eq. (2) explains over 58% of the variation in
site index. The standard error of estimate is 2.8 m
(12% of the average of the observed values). The
mean of absolute residuals is 2.2 m: this ®gure quanti
®es the average error associated with a single prediction.
The distributions of residuals against
predicted values of SI and predictors do not show
any signi®cant trend (Fig. 4). Partial regression leverage
plots evidence the signi®cant contribution of each
predictor (Fig. 5).