Distribution (left panel) and histogram (right panel) of RR1 residuals for the cubic fitting model. Left panel: the differences between the NMARCS theoretical fluxes and the fluxes obtained from our regression models as a function of the NMARCS theoretical fluxes. Right panel: the distribution of the residuals from the relation showed in the left panel. The absolute continuum flux is in 10⁵ erg cm^-2 s^-1 Å^-1 units.