An objective of DSP testing should be to ensure that any errors due to missed faults are infrequent compared to a circuit's intrinsic errors, such as overflow. A method is proposed for quantifying test quality for digital filters by measuring the risk associated with any untested faults. Techniques for finding upper bounds on fault activation rates under worst-case operating conditions are described. These techniques enable test designers to objectively discriminate significant missed faults from near-redundant faults, which are unlikely to be activated in normal operation of the device. This complements fault coverage as a measure of test quality, providing a means of locating high-risk missed faults even in very high coverage test regimes.