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It is now well established that the negative bias temperature instability (NBTI) mechanism alters both the mean and variance of the distribution of the PFET under stress. This effect has reliability implications to balanced analog circuits (e.g., current mirrors, differential pairs, etc.), as well as to SRAM cell stability. This paper presents a brief review of the understanding and models to date of the statistics and impacts of the NBTI-induced variation. This is followed by a critical examination of the actual NBTI-induced distributions and the accuracy of the normal approximations that have been used to date.