Lean Six Sigma Black Belt Certification Statistical Process Control (SPC) 4

Question 1

When calculating the process performance index Ppk, which measure of sigma is used?

Accepted Answer

Overall sample standard deviation of all individual values

Answer

Ppk uses the overall (long-term) standard deviation calculated from all individual data points, capturing both common and special cause variation.

Question 2

A Black Belt wants to detect a 1.5-sigma shift in the process mean quickly. Which chart would be MOST effective?

Accepted Answer

CUSUM or EWMA chart

Answer

CUSUM and EWMA charts are specifically designed to detect small to moderate process mean shifts (1–2 sigma) much faster than Shewhart charts.

Question 3

Which assumption is critical for control charts based on the normal distribution to be valid?

Accepted Answer

Observations must be independent and identically distributed

Answer

Control charts assume observations are independent and come from the same distribution; autocorrelation or non-stationarity can cause false signals and misleading limits.

Question 4

On an I-MR chart, the Moving Range chart monitors which source of variation?

Accepted Answer

Consecutive point-to-point variation (short-term variability)

Answer

The MR chart tracks the absolute difference between consecutive individual values, capturing short-term, point-to-point process variability.

Question 5

A process engineer discovers that subgroups on an X-bar chart were formed by collecting samples from multiple process streams mixed together. What problem does this cause?

Accepted Answer

Control limits that are too narrow, causing false alarms

Answer

Mixing samples from multiple streams within a subgroup inflates within-subgroup variation, narrowing control limits and producing excessive false alarms (a violation of rational subgrouping).

Lean Six Sigma Black Belt Certification Practice Test

Lean Six Sigma Black Belt Certification Practice Test

Lean Six Sigma Black Belt Certification Statistical Process Control (SPC) 4