FREE Certified Six Sigma Black Belt Statistical Process Control (SPC) Questions and Answers Flashcards
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Read the first 6 FREE Certified Six Sigma Black Belt Statistical Process Control (SPC) Questions and Answers flashcards as text
An I-MR chart shows the individual values chart in control, but the moving range chart has a point above the upper control limit. What should be investigated first?
Answer: Whether a single large jump occurred between two consecutive observations
The moving range chart measures point-to-point variation, so an out-of-control signal indicates an unusually large change between two consecutive individual measurements.
Which assumption is most critical for the valid interpretation of a standard X-bar and R control chart?
Answer: Observations within each subgroup must be independent
Independence of observations within subgroups is essential because autocorrelation inflates or deflates the estimated process variation, leading to incorrect control limits.
A Six Sigma Black Belt notices 8 consecutive points on one side of the center line on an X-bar chart. The process is considered:
Answer: Out of control due to a run indicating a likely mean shift
Eight consecutive points on the same side of the center line constitute a run, which is a pattern-based out-of-control signal indicating a probable shift in the process mean.
What is the Average Run Length (ARL) for a Shewhart control chart using only the 3-sigma rule when the process is in control?
Answer: 370 subgroups
With a false alarm probability of 0.0027 per point, the in-control ARL equals 1/0.0027 ≈ 370, meaning on average one false signal every 370 subgroups.
A CUSUM chart is preferred over a standard Shewhart chart in which situation?
Answer: When detecting small sustained shifts of 1.5 sigma or less
CUSUM charts accumulate deviations from the target value, making them significantly more sensitive than Shewhart charts for detecting small persistent shifts in the process mean.
When transforming non-normal data for use with standard SPC charts, which approach should be attempted first?
Answer: Apply a Box-Cox transformation to achieve approximate normality
The Box-Cox family of power transformations is the standard first approach for normalizing skewed continuous data so that conventional control chart assumptions are satisfied.