Statistical Process Control Flashcards

Which type of control chart is best suited for monitoring the number of defects per unit when the sample size varies?

Answer: u-chart

The u-chart tracks defects per unit and accommodates variable sample sizes, making it appropriate when inspection units differ in size.

A process is said to be in a state of statistical control when:

Answer: Only common cause variation is present

Statistical control means the process exhibits only common cause (random) variation, with no special causes influencing the output.

On an X-bar chart, what does the Western Electric Rule 2 (two of three points beyond 2 sigma) signal?

Answer: A potential shift in process mean requiring investigation

Two of three consecutive points beyond 2-sigma on the same side signals a probable shift in the process mean and should trigger investigation.

What is the primary purpose of the R-chart used alongside an X-bar chart?

Answer: Monitor within-subgroup variation (spread)

The R-chart monitors variability within subgroups, ensuring the spread of the process remains stable alongside tracking the mean.

A manufacturer samples 50 units each shift and records defective units. Which control chart is most appropriate?

Answer: np-chart

The np-chart is used for count of defective items with a constant sample size, making it ideal for this fixed-subgroup scenario.

In SPC, a 'run' of eight consecutive points on one side of the centerline most likely indicates:

Answer: A special cause that has shifted the process

Eight or more consecutive points on one side of the centerline is a run rule indicating a non-random pattern and probable special cause.

What does the D4 constant represent in control chart calculations for an X-bar and R chart?

Answer: Upper control limit factor for the R-chart

D4 is the factor multiplied by the average range (R-bar) to compute the upper control limit on the R-chart.