Results from the principal component analysis. Left: position of the observed galaxies in the PC base  [ PC1,PC2,PC3 ] . Different symbols refer to starbursts (stars), Seyferts (triangles) and LIRGs (squares). Galaxies with both AGN and LIRG properties are plotted as triangles. Numbers indicate different galaxies, as explained in Table 1. Values for the PAH equivalent widths are taken from Spoon et al. (2007). Infrared fluxes were obtained from the NASA/IPAC Extragalactic Database. Right: projection of the PC components onto the original base of observables. We consider only the first three PCs, since they account for 90% of the variance and thus contain most of the information in the dataset.