Statistical Process Control Flashcards

Process capability index Cp measures:

Answer: The ratio of specification width to process spread

Cp compares the total specification width (USL − LSL) to the process spread (6 sigma), indicating whether the process fits within tolerances.

A process has a Cp of 1.5 but a Cpk of 0.8. What does this indicate?

Answer: The process spread fits specs but it is not centered

A high Cp with a low Cpk means the process spread is narrow enough but the process mean is off-center, pushing output toward one specification limit.

Which SPC chart would you use to monitor the time between failures in a low-defect manufacturing environment?

Answer: g-chart

The g-chart (geometric chart) monitors the number of events or time between rare occurrences, suitable for very low defect rate processes.

Common cause variation in a process is best addressed by:

Answer: Fundamentally redesigning or improving the process system

Common causes are inherent to the system and can only be reduced through systematic process improvement or redesign, not by reacting to individual points.

What is the effect of increasing subgroup size (n) on the control limits of an X-bar chart?

Answer: Control limits narrow, making the chart more sensitive

Larger subgroup sizes reduce the standard error of the mean, narrowing control limits and making the chart more sensitive to process shifts.

In Phase I of control chart implementation, what is the primary objective?

Answer: Establishing stable baseline control limits from historical data

Phase I uses historical process data to identify and remove special causes, establishing stable control limits that represent the process baseline.

A Six Sigma process has approximately how many defects per million opportunities (DPMO)?

Answer: 3.4

A Six Sigma process, accounting for a 1.5-sigma mean shift, produces approximately 3.4 defects per million opportunities.