For obtaining a zero defect level, a high fault coverage with respect to the stuck-at fault model is often not sufficient as there are many defects that show a more complex behavior. In this paper, a method is presented for computing the occurrence probabilities of certain defects and the realistic fault coverage for test sets. The method is highly efficient as a pre-processing step is used for partitioning the layout and extracting the defects ranked in the order of their occurrence probabilities. The method was applied to a public domain library where defects causing a complex faulty behavior are possible. The occurrence probability of these faults was computed, and the defect coverage for different test sets was determined.