Control Phase: SPC Flashcards
7 cards from real Lean Six Sigma Black Belt Certification practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Control Phase: SPC flashcards as text
A Black Belt observes a trend of 6 consecutive points steadily increasing on an X-bar chart. Per Western Electric rules, this is:
Answer: A signal of a special cause warranting investigation
Six consecutive points in a monotonically increasing (or decreasing) trend is a Western Electric rule violation indicating a special cause such as tool wear or process drift.
The Ppk index differs from Cpk in that Ppk uses:
Answer: Overall process standard deviation calculated from all individual measurements
Ppk uses the overall (long-term) standard deviation from all data, while Cpk uses the within-subgroup (short-term) standard deviation estimated from the control chart.
When the process standard deviation σ is known, the control limits for an X-bar chart are set at:
Answer: X-double-bar ± 3σ/√n
With known σ, control limits are X-double-bar ± 3σ/√n, where n is the subgroup size, reflecting the standard error of the subgroup mean.
Rational subgrouping in SPC means grouping measurements so that:
Answer: Variation within subgroups is minimized while variation between subgroups captures process changes
Rational subgrouping minimizes within-subgroup variation (reflecting only short-term random noise) so that between-subgroup differences reveal meaningful process changes.
A process producing bolt diameters has been in statistical control for months. Specifications are 10mm ± 0.5mm. The process mean is 10.1mm with σ = 0.12mm. What is Cpk?
Answer: 1.11
Cpk = min[(USL-μ)/(3σ), (μ-LSL)/(3σ)] = min[(10.5-10.1)/0.36, (10.1-9.5)/0.36] = min[1.11, 1.67] = 1.11.
Which of the following is a key limitation of using only Cp and Cpk to assess process capability?
Answer: They assume the process data is normally distributed
Traditional Cp and Cpk calculations assume process data follows a normal distribution; non-normal data requires transformation or non-parametric capability methods.
In SPC, tampering (also called over-adjustment) occurs when an operator:
Answer: Adjusts the process in response to common cause variation
Tampering is adjusting a process in reaction to common cause variation (random noise), which actually increases overall process variability — a concept illustrated by Deming's funnel experiment.