SPC Process Variation & Capability 4

Question 1

A process produces 1,350 PPM defective. What is the approximate process sigma level?

Accepted Answer

4σ

Answer

Approximately 1,350 PPM corresponds to a 4-sigma process (with a 1.5σ shift assumption used in Six Sigma methodology).

Question 2

Which chart is most appropriate for monitoring individual measurements when subgrouping is not practical?

Accepted Answer

Individuals and Moving Range (I-MR) chart

Answer

The I-MR chart is used when data is collected one unit at a time and subgrouping is not feasible.

Question 3

What is the primary reason for collecting rational subgroups in SPC?

Accepted Answer

To ensure variation within subgroups reflects only common causes

Answer

Rational subgroups are designed so within-subgroup variation reflects only common causes, making between-subgroup shifts easier to detect.

Question 4

If a process has both its mean and standard deviation change simultaneously, which chart would detect this situation most completely?

Accepted Answer

Both the X-bar and R charts used together

Answer

The X-bar chart detects mean shifts while the R chart detects changes in variation; using both together provides complete process monitoring.

Question 5

A process with high Cpk but frequent customer complaints most likely suffers from:

Accepted Answer

Specification limits set too wide relative to actual customer needs

Answer

If Cpk is high but complaints occur, the specification limits may be wider than what customers actually need, masking true quality problems.

(SPC) Statistical Process Control Certification Practice Test