It has been apparent for some time that comparison of continuum and emission-line light curves of AGN may enable determination of the dimensions and structure of the Broad Line Region (BLR)(see Peterson 1988, and references therein). Observations to this end (Antonucci and Cohen 1983, Peterson et al. 1985, Clavel et al. 1987) and their analysis using cross-correlation to find the time lag between line and continuum variations (Gaskell and Sparke 1986), indicate BLR sizes an order of magnitude smaller than allowed by standard photoionization models for several AGN. Gaskell and Peterson (1987) analyzed errors in the cross-correlation method as applied to AGN time series in general and specifically to the Seyfert galaxy Akn120. In order to generalize and extend their analysis we investigated the significance of BLR sizes derived by cross-correlation under different model assumptions and observational circumstances (Maoz and Netzer 1988).