For all models the p-value for lack of fit was above 0.10, indicating that the models were well adjusted (90% confidence interval). The values of R 2 adj are a measure of the amount of variation explained by the models and they were all above 0.71. These values should be close to R 2 , but a difference of up to 0.2 is considered acceptable. However, the R 2 predicted values were very low (except for STDCapryol). These values, as well as the PRESS (predicted residual sum of squares) ones, were calculated using the leave-one-out method.