This article describes several deficiencies with traditional assessments of negative bias temperature instability (NBTI) in pMOS transistors and proposes methods for handling them. These effects include: (a) a decrease in the rate of degradation over time, (b) a deviation of the stress bias dependence of NBTI lifetime from simple analytical models, (c) partial dynamic recovery of apparent NBTI degradation after interruption of stress, and (d) errors well beyond what might naively be expected in lifetime extrapolation due to uncertainties in measurement and modeling of NBTI. These errors can even be several orders of magnitude. If these effects are not adequately considered in NBTI characterization, assessment, benchmarking, and optimization, they could lead excessive expense in product reliability evaluation or, worse, to unanticipated, costly field reliability problems.