The statistical definition of Six Sigma allows for 3.4 defects per million. This is achieved by a Cpk index of
A) 0
B) 1
C) 1.33
D) 1.67
E) 2

Question

Quizplus · Accepted Answer

The Cpk index is a measure of process capability. It measures how well the process is able to produce items within the specification limits. The larger the Cpk index, the better the process capability. The formula for Cpk index is Cpk = min[(USL - X̄)/3σ, (X̄ - LSL)/3σ], where USL is the upper specification limit, X̄ is the process mean, LSL is the lower specification limit, and σ is the process standard deviation. For Six Sigma, the process capability needs to be such that there are only 3.4 defects per million items produced. This translates to a Cpk index of 2. Since the process capability needs to be double the standard deviation away from the limits, the Cpk index of 2 means that the process mean is 6σ away from the specification limits. Therefore, the answer is E, 2.

The Statistical Definition of Six Sigma Allows for 3