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In this letter, a strategy to minimize the error in lifetime projections using a positive bias temperature instability (PBTI) test has been proposed. Two distinctly different projection slopes were observed in a plot of time to failure versus oxide electric field. A small slope in the high-field region, which means weaker electric field dependence, led to an underestimation of lifetime. This result was attributed to a filled trap cluster at a specific trap energy level, locally reducing the oxide electric field. Thus, different lifetimes can be projected depending on stress bias. Maintaining a PBTI stress bias range below this trap energy level is recommended for accurate projections.