SPC Control Charts & Analysis 3

Question 1

What is the primary purpose of the R chart in an X-bar & R chart pair?

Accepted Answer

Detect shifts in subgroup variability

Answer

The R chart monitors within-subgroup variability (range), ensuring the spread of the process remains stable before interpreting the X-bar chart.

Question 2

Zone C on a control chart (using the Western Electric zone rules) corresponds to which sigma band?

Accepted Answer

Within ±1 sigma

Answer

Zone C is the innermost zone, spanning within ±1 sigma of the centerline, where approximately 68% of points should fall.

Question 3

Which attribute control chart is used when subgroup sizes are constant and you are counting the number of defective units (not defects)?

Accepted Answer

np-chart

Answer

The np-chart tracks the actual count of defective units per subgroup and requires constant subgroup size.

Question 4

A control chart shows 15 consecutive points alternating above and below the centerline. This pattern most likely indicates:

Accepted Answer

Systematic over-adjustment (tampering)

Answer

Alternating zigzag patterns often indicate tampering, where operators over-correct the process in response to each reading.

Question 5

For an X-bar chart with subgroup size n=5, which D4 constant value is used to compute the UCL of the R chart?

Accepted Answer

2.114

Answer

For n=5, the D4 constant is 2.114, so the UCL for the R chart equals D4 × R-bar = 2.114 × R-bar.

(SPC) Statistical Process Control Certification Practice Test