Yield and quality control

Which formula gives the process capability index Cp (when the process mean coincides with the center of the specification)? (σ: process standard deviation)

  1. A(Upper spec limit − lower spec limit) ÷ 6σ
  2. BMean ÷ σ
  3. Cσ ÷ mean
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Answer: A. (Upper spec limit − lower spec limit) ÷ 6σ

Cp is the specification width divided by the process variation (6σ), indicating how much margin the process has in meeting the specification. Generally, a Cp of 1.33 or more is considered adequate process capability. When the mean is off the specification center, Cpk, which accounts for the offset, is used.

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