Statistical Process Control Flashcards

Which type of variation is inherent to a stable process and cannot be eliminated without changing the process itself?

Answer: Common cause variation

Common cause variation is the natural, random variation built into the process design, and reducing it requires fundamental process changes rather than firefighting.

What does an OC (Operating Characteristic) curve display in acceptance sampling?

Answer: The probability of accepting a lot at various quality levels

An OC curve plots the probability of lot acceptance against the actual incoming lot quality (fraction defective), helping evaluate sampling plan performance.

In SPC, a 'run of 8 consecutive points on one side of the centerline' is an example of:

Answer: A signal of special cause variation

Eight consecutive points on one side of the centerline is a classic run rule that signals a process shift or special cause, as the probability of this occurring by chance is very low.

Short-term process capability is typically measured using data collected over what period?

Answer: A single brief study period under stable conditions

Short-term capability (Cp/Cpk) is measured during a controlled study with minimal time-to-time variation, representing the best the process can achieve.

What statistical distribution is typically assumed for attribute control charts such as the p-chart?

Answer: Binomial distribution

The p-chart is based on the binomial distribution, which models the probability of a certain number of nonconforming items in a sample when each item is independently classified as pass or fail.

Which index would you use if a process has a one-sided specification limit only?

Answer: CPU or CPL individually

When only one specification limit exists, you use either CPU (upper) or CPL (lower) alone, since Cp requires both limits and Cpk takes the minimum of both.