Control: SPC & Control Charts

Which control chart is most appropriate for monitoring individual measurements where the natural subgroup size is one?

• A Individuals and Moving Range (I-MR) chart ✓
• B p-chart
• C c-chart
• D X-bar and R chart

Answer: When data are collected as single observations rather than subgroups, the Individuals and Moving Range (I-MR) chart is the correct choice.

A team tracks the proportion of defective items in daily samples whose sizes vary from day to day. Which control chart should be used?

• A X-bar and S chart
• B np-chart
• C c-chart
• D p-chart ✓

Answer: The p-chart monitors the proportion of defective units and accommodates varying subgroup sizes, unlike the np-chart which requires a constant sample size.

Which chart is designed to monitor the count of defects per inspection unit when the sample size (area of opportunity) is constant?

• A p-chart
• B X-bar and R chart
• C I-MR chart
• D c-chart ✓

Answer: The c-chart tracks the number of defects (nonconformities) per unit when the area of opportunity is constant, modelled by the Poisson distribution.

On a standard Shewhart control chart, the upper and lower control limits are conventionally placed at:

• A Plus or minus 1 standard deviation from the centreline
• B Plus or minus 2 standard deviations from the centreline
• C Plus or minus 3 standard deviations from the centreline ✓
• D At the customer's upper and lower specification limits

Answer: Shewhart control limits are set at plus or minus 3 sigma from the centreline, capturing about 99.73% of common-cause variation.

A single point falling outside the control limits on a control chart is a signal of:

• A Common-cause (random) variation that should be ignored
• B A special (assignable) cause of variation that warrants investigation ✓
• C An automatic need to recalculate the specification limits
• D Improved process capability

Answer: A point beyond the 3-sigma control limits indicates a special (assignable) cause, signalling the process has changed and should be investigated.

Which statement correctly distinguishes control limits from specification limits?

• A Control limits are calculated from process data (the voice of the process); specification limits come from customer or design requirements ✓
• B Control limits and specification limits are always identical
• C Specification limits are always plus or minus 3 sigma of the process mean
• D Control limits are set by the customer; specification limits are calculated from process data

Answer: Control limits reflect the voice of the process and are computed from actual process variation, whereas specification limits express customer or design requirements and are independent of the process.