Which of these are correct if Cpk Upper is 2.0 and Cpk Lower is 1.0?

Question

Options

• AThe process is not stable.
• BThe process is shifted to the left.
• CCpk must be reported as 1.0.
• DThe process Mean is 1.5.

• A
  5% (2)
• B
  2% (1)
• C
  79% (33)
• D
  14% (6)

Explanation

Cpk is always reported as the minimum of the upper and lower capability indices, so min(2.0, 1.0) = 1.0 - the tighter, more at-risk side governs, making C correct. Option A is wrong because process stability is assessed through control charts and time-ordered data, not Cpk values; a process can be statistically unstable yet still yield a calculable Cpk. Option B is a distractor: while CPU > CPL does suggest the mean has more clearance on the upper side (consistent with a leftward shift toward the lower spec), Cpk ratios are dimensionless - you cannot confirm a directional label like "left" without the actual mean and specification values. Option D exploits a common arithmetic mistake - (2.0 + 1.0) / 2 = 1.5 - but Cpk values are ratios of distances to sigma, not the process mean itself.

Memory tip: Think of Cpk as a chain - it's only as strong as its weakest link. Always take the minimum.

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