CQI Statistical Process Control Flashcards

What is the primary purpose of a control chart?

Answer: To distinguish between common cause and special cause variation

Control charts are used to monitor processes over time and distinguish between normal (common cause) variation and abnormal (special cause) variation.

Which control chart is used to monitor the proportion of defective items in a sample?

Answer: p-chart

The p-chart monitors the proportion of nonconforming units in samples and is an attribute control chart for fraction defective.

What do the Upper Control Limit (UCL) and Lower Control Limit (LCL) represent on a control chart?

Answer: The boundaries within which process variation is expected due to common causes (±3σ)

UCL and LCL are calculated at ±3 standard deviations from the process mean and define the expected range of common cause variation.

Which of the following is a signal of special cause variation on a control chart?

Answer: Eight consecutive points on the same side of the centerline

Eight consecutive points on the same side of the centerline (a Nelson/Western Electric rule) indicate a process mean shift, signaling special cause variation.

An X-bar and R chart monitors:

Answer: Variable (continuous) data — subgroup means and ranges

The X-bar and R chart monitors variable (continuous) data by tracking subgroup averages (X-bar) and ranges (R) to detect process shifts and spread changes.

What is the typical subgroup size used with an X-bar and R chart?

Answer: 2 to 5

X-bar and R charts are typically used with subgroup sizes of 2 to 5 measurements, which is sufficient to estimate variation and detect process shifts.