This article develops a regional seismic discrimination method using information inherent in phase amplitudes that are unmeasurable due to small signal amplitudes and high noise levels. The method, quadratic negative evidence discrimination (QNED), is an enhancement to the teleseismic techniques proposed by Elvers (1974) and is extended to regional discrimination. The method presented in this article is developed for a single seismic station and makes use of the empirical evidence in the regional Pg versus Lg discriminant (see Pomeroy et al., 1983). We develop the equations necessary to compute the station-specific, missed-explosion, and false-alarm error rates. These error rates depend on the required minimum signal-to-noise ratio (S/N) and can be adjusted, within limits, to desired levels. We also show that these equations are an accurate assessment of the errors in seismic discrimination. We propose that many of the current approaches to assessing seismic discrimination errors are often overly optimistic. For some applications, this disparity can be significant. For an application to Western China regional data [Pg versus Lg (1.5 to 3 Hz)], a widely used estimate of the missed-explosion error rate is 20%, leading to the perception that explosions can be identified with an accuracy rate of 80%. A proper accounting of the missed-explosion error rate, using QNED, shows that explosions can be identified with accuracy rate of only 73%.