SPC Process Variation & Capability 3

Question 1

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

Accepted Answer

Within a brief window capturing mainly within-subgroup variation

Answer

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

Question 2

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

Accepted Answer

d2

Answer

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

Question 3

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

Accepted Answer

Low process variation but the mean is significantly off-center

Answer

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.

Question 4

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

Accepted Answer

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

Answer

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.

Question 5

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

Accepted Answer

Cpk

Answer

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

(SPC) Statistical Process Control Certification Practice Test