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While the matched filter/equalizer combination has been known for years to produce "optimal" demodulators whose performance is well known, it can be difficult to predict the performance of a "nonoptimal" demodulator suffering from mismatched filtering, or poor synchronization. This paper demonstrates how to use the mean square symbol estimation error as a means of evaluating binary phase shift keyed (BPSK) demodulator performance under "nonoptimal" conditions. By applying the formulas presented, it is possible to predict both mean square error, as well as the signal to noise ratio at the output of the demodulator. These metrics are then used to predict losses caused by nonoptimal filtering, and in some cases to predict bit error performance. In every case, the MSE estimate is shown to correctly predict system performance.