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This paper describes a new procedure to estimate the delay-dependent switching activities in CMOS combinational circuits. The procedure is based on analytic and statistical approaches to take advantage of their time-efficiency over conventional event-driven simulation tools. For this study, combinational circuits driven by discrete-time logic signals are considered. By focusing on a specific class of combinational circuits, the transitional effects can be analyzed more accurately by considering some of the delay effects neglected in previous studies, Also, to model the delay-dependent effects, statistical properties such as the pattern probability, the propagation probability, and the distribution of the propagation delay of switching activities are defined and evaluated. The simulation results on benchmark circuits indicate that the proposed procedure significantly speeds up the estimation process in comparison to the conventional event-driven simulators. The reliability issues in the aspect of switching activities are briefly discussed.