Process Variation & Capability Flashcards

A process has a Cpk of 0.85. What does this indicate about the process?

Answer: The process is not capable and may be producing defects

A Cpk below 1.0 indicates the process is not capable and is likely producing output outside specification limits.

Which type of variation is represented by a single point falling outside the control limits on an X-bar chart?

Answer: Special cause variation

Points outside control limits are a signal of special cause (assignable cause) variation that requires investigation.

The Pp index differs from Cp in that Pp uses:

Answer: The overall standard deviation of all collected data

Pp uses the overall standard deviation calculated from all data points, while Cp uses the short-term within-subgroup estimate.

In a process capability study, what does the 'voice of the customer' refer to?

Answer: The specification limits set by customer requirements

The voice of the customer is represented by specification limits (USL and LSL) that define acceptable product boundaries.

If USL = 50, LSL = 30, and the process mean is 38, which formula gives the correct Cpk?

Answer: min[(50-38)/3σ, (38-30)/3σ]

Cpk is the minimum of the upper and lower capability ratios, reflecting the worst-case distance to either spec limit.

A process exhibits a 'sawtooth' pattern on a control chart. This most likely indicates:

Answer: Over-adjustment or tampering with the process

A sawtooth (alternating high-low) pattern is a classic sign of over-adjustment, where operators correct perceived error and create opposite swings.

What is the relationship between process spread and the specification width when Cp = 1.33?

Answer: The process spread is 75% of the specification width

Cp = 1.33 means 6σ = specification width / 1.33, so the process uses about 75% of the available specification space.