What quality levels did Smith determine as necessary, including a tolerance for shifts?

The determination of quality levels that include a tolerance for shifts is reflected in the concept of process capability and the design of Six Sigma methodologies. Specifically, when Smith identifies quality levels with a tolerance for shifts, he is referring to a process capable of functioning at what is often called the Six Sigma level, which typically corresponds to a process performance centered accurately with respect to its specifications while accommodating potential shifts.

The inclusion of +/- 1.5 sigma in addition to +/- 3 sigma acknowledges that real-world processes experience variations over time, and this can lead to a shift in the process mean. The +/- 3 sigma represents the control limits for process variation, while the +/- 1.5 sigma accommodates an expected drift or shift that might happen in practice, ensuring that the process remains within desired quality levels even when this drift occurs.

This approach is critical in achieving a high level of quality assurance, as it proactively addresses the variability that may not be controlled under static conditions, thereby ensuring that the process remains capable, robust, and responsive to changes or fluctuations in performance over time.