  Reverberation mapping offers one of the best techniques for studying the
inner regions of QSOs. It is based on cross-correlating continuum and
emission-line light curves. New time-resolved optical surveys will produce well
sampled light curves for many thousands of QSOs. We explore the potential of
stacking samples to produce composite cross-correlations for groups of objects
that have well sampled continuum light curves, but only a few (~2)
emission-line measurements. This technique exploits current and future
wide-field optical monitoring surveys (e.g. Pan-STARRS, LSST) and the
multiplexing capability of multi-object spectrographs (e.g. 2dF, Hectospec) to
significantly reduce the observational expense of reverberation mapping, in
particular at high redshift (0.5 to 2.5).
  We demonstrate the technique using simulated QSO light curves and explore the
biases involved when stacking cross-correlations in some simplified situations.
We show that stacked cross correlations have smaller amplitude peaks compared
to well sampled correlation functions as the mean flux of the emission light
curve is poorly constrained. However, the position of the peak remains intact.
We find there can be `kinks' in stacked correlation functions due to different
measurements contributing to different parts of the correlation function.
  Using the Pan-STARRS Medium-Deep Survey (MDS) as a template we show that
cross-correlation lags should be measurable in a sample size of 500 QSOs that
have weekly photometric monitoring and two spectroscopic observations. Finally
we apply the technique to a small sample (42) of QSOs that have light curves
from the MDS. We find no indication of a peak in the stacked cross-correlation.
A larger spectroscopic sample is required to produce robust reverberation lags.
