Lean Six Sigma Black Belt Certification Control Phase: SPC 2

Question 1

On an X-bar chart, a process shows 8 consecutive points all above the centerline. What does this pattern indicate?

Accepted Answer

A shift in the process mean

Answer

Eight consecutive points on the same side of the centerline is a Western Electric run rule indicating a sustained shift in the process mean.

Question 2

Which SPC chart is most appropriate for monitoring the number of defects per unit when the sample size varies?

Accepted Answer

u chart

Answer

The u chart monitors defects per unit and accommodates variable sample sizes, unlike the c chart which requires constant sample size.

Question 3

Process capability index Cpk differs from Cp in that Cpk:

Accepted Answer

Accounts for process centering as well as spread

Answer

Cpk accounts for both process spread and the location of the process mean relative to specification limits, penalizing off-center processes.

Question 4

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?

Accepted Answer

Two distinct process streams mixed in sampling

Answer

A consistent alternating pattern often indicates two process streams (e.g., two machines or shifts) being sampled alternately within subgroups.

Question 5

The Average Run Length (ARL) for a Shewhart X-bar chart with 3-sigma limits when the process is in control is approximately:

Accepted Answer

370

Answer

For a 3-sigma control chart in control, the ARL is approximately 370, meaning a false alarm occurs on average every 370 subgroups.

Lean Six Sigma Black Belt Certification Practice Test