We examined the reliability of three traditional P50 auditory event-related potential indices under paired-click conditions: (a) the conditioning response (C), (b) the testing response (T), and (c) the testing to conditioning suppression ratio (T/C). Three alternative indices, (a) the (C−T) difference, (b) the (C−T)/(C+T) adjusted difference, and (c) the (T−T) residualized difference, where T′ is the regression of T on C, were also studied. The N 100 wave was used as a generalizability check. Although C and T amplitudes were reliably were reliably measured by traditional means, the T/C suppression ratio was not. for psychometric reasons that are described, the reliability of the suppression ratio is undermined principally by the correlation between C and T. The C−T difference score is a promising alternative to the unreliable T/C suppression ratio. Theoretical consequences of changed metrics are discussed.