Principal Component Analysis (PCA) was applied as an exploratory
tool of data structure (Vandeginste et al.,1997). This statistical
multivariate technique is, usually, the first step in data exploration:
PCA defines new variables, consisting of linear combinations of the
original ones, in such a way that the first axis is in the direction
containing most variation. Every subsequent new variable (principal
component) is orthogonal to the previous one, but again in the
direction containing most of the remaining variance