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Statistical Fault Analysis, or Stafan, is proposed as an alternative to fault simulation of digital circuits. This method defines Controllabilities and observabilities of circuit nodes as probabilities estimated from signal statistics of fault-free simulation. Special Procedures deal with these quantities at fanout and feedback nodes. The computed probabilities are used to derive unbiased estimates of fault detection probabilities and overall fault coverage for the given set of input vectors. Among Stafan's advantages, fault coverage and the undetected fault data obtained for actual circuits are shown to agree within five percent of fault simulator results, yet CPU time and memory demands fall far short of those required in fault simulation. The Computational complexity added to a fault-free simulator by Stafan grows only linearly with the number of circuit nodes.