Why does Six Sigma assume a process that is shifted by 1.5 sigma from the midpoint of the specification interval? What effect does this have on the quality level implied by Six Sigma?

Six Sigma is a set of techniques and tools for process improvement. The primary goal of Six Sigma is to reduce process variation so that virtually all products or services meet the customer's specifications. Six Sigma has a statistical basis, with the "six" in the name relating to the number of standard deviations between the process mean and the specification limits. A Six Sigma process has a defect rate of 3.4 defects per million opportunities, which corresponds to 99.9997% of the products or services meeting the customer's specifications.

In real-world processes, it is common for the process mean to drift or shift over time due to factors such as machine wear, environmental changes, or other variables. The process shift is the difference between the actual process mean and the midpoint of the specification interval. In other words, it represents how much a process deviates from the ideal scenario where the process mean is exactly at the center of the specification limits.

Six Sigma methodology assumes a process shift of 1.5 sigma from the midpoint of the specification interval. This assumption is based on observational data and empirical studies, which suggest that most processes tend to shift by 1.5 sigma over time. The 1.5-sigma shift provides a buffer or margin, accounting for the inevitable process drift and ensuring that product or service quality remains high even when the process mean moves away from the midpoint.

Incorporating the 1.5-sigma shift assumption allows Six Sigma to maintain an exceptionally high quality level even in the presence of process drift. By accounting for this likely shift, Six Sigma practitioners can more accurately predict and control process performance, ensuring that products or services will maintain a very low defect rate even with some process variation. Without the 1.5-sigma shift assumption, processes that achieve Six Sigma quality levels might not be as resilient to process drift, leading to a higher likelihood of defects and deviations from customer requirements. In conclusion, Six Sigma assumes a process that is shifted by 1.5 sigma from the midpoint of the specification interval to account for the common drift in processes over time. This assumption allows for better predictive modeling and control of process performance, ensuring that the Six Sigma methodology leads to high-quality products and services with exceptionally low defect rates even in the presence of process variability.