Process Variation & Capability Flashcards

A short-term capability study is best described as measuring variation:

Answer: Within a brief window capturing mainly within-subgroup variation

Short-term capability (Cp/Cpk) is measured over a brief period to capture within-subgroup variation, minimizing between-subgroup sources.

When calculating the estimated process standard deviation using the average range method, which constant is used?

Answer: d2

The constant d2 is used to convert the average range (R-bar) to an estimate of the process standard deviation: σ̂ = R̄/d2.

A process with Cp = 1.5 but Cpk = 0.9 most likely has which characteristic?

Answer: Low process variation but the mean is significantly off-center

When Cp is high but Cpk is low, the process spread is narrow but the mean is shifted away from the target, reducing one-sided capability.

Which of the following is a Western Electric run rule for detecting special causes?

Answer: Two out of three consecutive points beyond 2-sigma on the same side

Two out of three consecutive points in Zone A (beyond 2σ) on the same side is a standard Western Electric run rule signaling a special cause.

In a unilateral tolerance scenario (only one specification limit), which capability index is most appropriate?

Answer: Cpk

Cpk is appropriate for unilateral tolerances because it measures the distance to the relevant single specification limit.

What effect does reducing subgroup size have on an X-bar chart's sensitivity to process shifts?

Answer: Decreases sensitivity to small shifts

Smaller subgroups widen control limits (higher variation in X-bar), reducing the chart's ability to detect small process mean shifts.

The Cpm index (Taguchi capability index) incorporates which additional factor compared to Cp?

Answer: The deviation of the process mean from the nominal target

Cpm penalizes processes whose mean deviates from the nominal target, even if the mean is within specification limits.