Control Charts & Analysis Flashcards

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

Answer: Detect shifts in subgroup variability

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

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

Answer: Within ±1 sigma

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

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

Answer: np-chart

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

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

Answer: Systematic over-adjustment (tampering)

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

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

Answer: 2.114

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

What distinguishes a u-chart from a c-chart?

Answer: u-chart allows variable inspection area; c-chart requires constant area

The u-chart expresses defects per unit and accommodates variable inspection areas or subgroup sizes, whereas the c-chart requires a constant area of opportunity.

On an I-MR chart, the Moving Range is calculated as:

Answer: The absolute difference between consecutive individual values

The Moving Range is |Xi − Xi-1|, the absolute value of the difference between each pair of consecutive individual measurements.