Measurements of EW(Hδ) and D_n(4000) of the coadded spectra of the high-L_TIR (circles), moderate-L_TIR (diamonds), and low-L_TIR subsamples corrected (filled light gray symbols) and uncorrected (filled dark gray symbols) for the effects of extinction. These measurements are plotted against the backdrop of equivalently measured quantities from CB07 synthetic spectra for various time steps using various models. For models with secondary bursts, these bursts occur 2 Gyr after the initial burst. No extinction is applied to the CB07 models and the models employ a Salpeter (1955) IMF, identical to the IMF used for all SFR calculations. The decreasing starburst strength relative to the already established stellar mass for galaxies of decreasing L_TIR suggest that the LIRG population is experiencing rejuvenated star formation at moderate levels while the average ULIRG is undergoing either its initial starburst or an extremely vigorous (i.e., 100% by mass) rejuvenated star-formation event.