Fig. 4.

Receiver–operator curves of the various logistic regression models. Black line, simple model built and tested on training set; gray lines, simple models built on the bootstrapped set and tested on training set; blue line, simple model built on training set and tested on validation set; brown line, model built solely by using shoot Fe concentrations and tested on the full training set; red dot, cutoff level chosen for further studies; dotted line, unity, a model with no predictive ability.