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
On an X-bar chart, a process shows 8 consecutive points all above the centerline. What does this pattern indicate?
Answer: A shift in the process mean
Eight consecutive points on the same side of the centerline is a Western Electric run rule indicating a sustained shift in the process mean.
Which SPC chart is most appropriate for monitoring the number of defects per unit when the sample size varies?
Answer: u chart
The u chart monitors defects per unit and accommodates variable sample sizes, unlike the c chart which requires constant sample size.
Process capability index Cpk differs from Cp in that Cpk:
Answer: Accounts for process centering as well as spread
Cpk accounts for both process spread and the location of the process mean relative to specification limits, penalizing off-center processes.
A control chart shows points within control limits but with a distinct sawtooth pattern alternating high-low-high-low. What is the most likely cause?
Answer: Two distinct process streams mixed in sampling
A consistent alternating pattern often indicates two process streams (e.g., two machines or shifts) being sampled alternately within subgroups.
The Average Run Length (ARL) for a Shewhart X-bar chart with 3-sigma limits when the process is in control is approximately:
Answer: 370
For a 3-sigma control chart in control, the ARL is approximately 370, meaning a false alarm occurs on average every 370 subgroups.
When establishing control limits for a new X-bar and R chart, the standard approach is to collect at least how many rational subgroups?
Answer: 20-25
A minimum of 20–25 subgroups is the standard recommendation to establish reliable control limits with adequate statistical stability.
A process has Cp = 1.5 but Cpk = 0.8. What does this tell you?
Answer: The process has adequate spread but is significantly off-center
Cp = 1.5 shows the process spread fits within specs, but Cpk = 0.8 reveals the process mean is shifted toward or beyond a spec limit.