  The cross-correlation function (CCF) is commonly employed in the study of
AGN, where it is used to probe the structure of the broad line region by line
reverberation, to study the continuum emission mechanism by correlating
multi-waveband light curves and to seek correlations between the variability
and other AGN properties. The z -transformed discrete correlation function
(ZDCF) is a new method for estimating the CCF of sparse, unevenly sampled light
curves. Unlike the commonly used interpolation method, it does not assume that
the light curves are smooth and it does provide errors on its estimates. The
ZDCF corrects several biases of the discrete correlation function method of
Edelson & Krolik (1988) by using equal population binning and Fisher's z
-transform. These lead to a more robust and powerful method of estimating the
CCF of sparse light curves of as few as 12 points. Two examples of light curve
analysis with the ZDCF are presented. 1) The ZDCF estimate of the
auto-correlation function is used to uncover a correlation between AGN
magnitude and variability time scale in a small simulated sample of very sparse
and irregularly sampled light curves. 2) A maximum likelihood function for the
ZDCF peak location is used to estimate the time-lag between two light curves.
Fortran 77 and 95 code implementations of the ZDCF and the maximum likelihood
peak location (PLIKE) algorithms are freely available (see
http://www.weizmann.ac.il/weizsites/tal/research/software/).
