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A new type of fault dictionary called a same/different fault dictionary is described. The same/different fault dictionary is similar to a pass/fail fault dictionary in that it contains a single bit b i,j for every modelled fault f i and test vector t j. However, in a pass/fail fault dictionary, b i,j is determined by comparing the output vector of the faulty circuit with the output vector of the fault-free circuit; whereas in a same/different fault dictionary, b i,j is determined by comparing the output vector of the faulty circuit with a preselected output vector called a baseline output vector. By selecting appropriately the baseline output vectors for all the test vectors, it is possible to obtain increased diagnostic resolution with a same/different fault dictionary compared with a pass/fail fault dictionary. A heuristic procedure for selecting baseline output vectors is described and experimental results are presented.