Statistical Static Timing Analysis (SSTA) is becoming necessary, but has not been widely adopted due to various weaknesses. In this paper, we address one of the challenging problems in SSTA: computation of correlations between cell delays. With the help of conditional moments, cell-to-cell and path-to-path delay correlations are computed by propagating iteratively means and variances of cell delays. This technique of computation allows considering the effects of cell topology, input slope and output load values. Numerical results are presented to quantify its accuracy.