Control Charts & Analysis Flashcards

Which Western Electric rule is violated when two out of three consecutive points fall beyond 2 sigma on the same side of the centerline?

Answer: Rule 2

Western Electric Rule 2 flags two out of three consecutive points beyond 2 sigma on the same side as a signal of special cause variation.

On an X-bar chart, the Lower Control Limit (LCL) formula using the A2 constant is:

Answer: X-double-bar − A2 × R-bar

The LCL for an X-bar chart is calculated as X-double-bar minus A2 times R-bar, where R-bar is the average range.

A p-chart is most appropriate when monitoring which type of data?

Answer: Proportion of defective items in variable-size subgroups

A p-chart monitors the proportion (fraction) of defective items and accommodates variable subgroup sizes.

What does a steadily increasing trend of 7 or more consecutive points on a control chart indicate?

Answer: A special cause trend worth investigating

Seven or more consecutive points trending in one direction is a classic special cause signal indicating a systematic shift or drift in the process.

Which control chart pair is used for variables data when subgroup size is 1?

Answer: Individuals and Moving Range (I-MR) chart

The Individuals and Moving Range (I-MR) chart is designed for subgroup size of n=1, where no within-subgroup range can be computed.

In a c-chart, what assumption is made about the distribution of defect counts?

Answer: Poisson distribution

The c-chart assumes defect counts follow a Poisson distribution, which is appropriate for rare events in a constant area of opportunity.

When the process average shifts by 1.5 sigma, the long-term defect rate for a 6-sigma process is approximately:

Answer: 3.4 PPM

A 6-sigma process with the standard 1.5-sigma long-term shift yields approximately 3.4 defects per million opportunities.