Certified Six Sigma Black Belt Statistical Process Control (SPC) Questions and Answers Flashcards
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Read the first 6 Certified Six Sigma Black Belt Statistical Process Control (SPC) Questions and Answers flashcards as text
A Black Belt is setting up a control chart for a process where individual measurements follow a non-normal distribution. Which approach is most appropriate?
Answer: Transform the data to achieve approximate normality before charting
When individual measurements are non-normal, applying a data transformation (such as Box-Cox) to achieve approximate normality allows valid use of individuals charts with standard control limit calculations.
What is the primary purpose of rational subgrouping in SPC?
Answer: To minimize the variation within subgroups so that between-subgroup variation represents assignable causes
Rational subgrouping is designed so that variation within each subgroup reflects only common cause variation, making any special cause variation appear as differences between subgroups on the control chart.
A process is monitored using a p chart with variable subgroup sizes. The control limits appear as a staircase pattern. What causes this?
Answer: Different subgroup sizes produce different standard error values, resulting in different control limit widths
On a p chart with variable subgroup sizes, each subgroup's control limits are calculated using its own sample size in the standard error formula, producing wider limits for smaller subgroups and narrower limits for larger ones.
Which of the following is a key assumption for calculating a valid Cpk index?
Answer: The data must come from a process that is in statistical control
Cpk is a meaningful capability measure only when the process is in statistical control, because an unstable process has unpredictable variation that makes capability indices unreliable.
A CUSUM chart signals an out-of-control condition. Compared to a Shewhart chart, what type of process shift is the CUSUM chart most effective at detecting?
Answer: Small persistent shifts of 0.5 to 2 sigma
The CUSUM (Cumulative Sum) chart accumulates deviations from a target value, making it highly sensitive to small persistent shifts in the range of 0.5 to 2 sigma that Shewhart charts often miss.
When constructing an X-bar and S chart instead of an X-bar and R chart, what is the primary reason for choosing the S chart?
Answer: The S chart is more accurate when subgroup sizes are large, typically greater than 8 or 10
When subgroup sizes exceed approximately 8 to 10, the range becomes a less efficient estimator of variability, and the sample standard deviation (S) provides a more accurate and statistically efficient estimate.