poor for AGN variability studies. We therefore base our flux calibration on a scale defined by the observed flux in the prominent narrow \[O[iii]{}\]$\lambda\lambda4959,$ 5007 doublet. These lines originate in a low-density region that is more spatially extended than the BLR or the continuum source, which for practical purposes can be regarded as point sources. The larger light-travel time and long recombination time ensure that any narrow-line variations will occur only over much longer time scales than of interest in this experiment. We therefore assume that the \[O[iii]{}\] lines are constant in flux, and use these to scale each spectrum. All spectra are scaled to a constant flux of $F(\mbox{\o5007}) = (3.91 \pm 0.12) \times 10^{-13}$, which is based on the mean of ten spectra from data set A that were obtained under photometric observing conditions during the 1996 observing season. The ø5007 flux measured from photometric spectra from subsequent years supports our assumption that this value can be assumed to be constant over the time scales of interest. All of the spectra are adjusted in flux to have this value of $F(\mbox{\o5007})$ by employing the spectral scaling software described by van Groningen & Wanders (1992). The process is as 