Since the variables analyzed in the present work have different
scales, before calculating the PCA models they were pre-processed
using autoscaling in order to give a priori the same importance
to each original variable. The number of significant PCs, i.e., the
number of PCs bearing useful information, indicates the number of
fundamentally different properties exhibited by the data set; in the
present work, two significant PCs were selected for each analyzed
dataset.