Fundamentals Flashcards

The Western Electric (WECO) rules are used in SPC to:

Answer: Detect non-random patterns in control chart data

WECO rules are a set of pattern-recognition tests applied to control charts to detect special causes beyond simple out-of-limit points.

What is 'over-adjustment' (also called tampering) in the context of SPC?

Answer: Making process adjustments in response to common cause variation

Tampering occurs when operators make unnecessary adjustments to a process reacting to common cause variation, which actually increases overall variation.

For an X-bar and R chart, after how many subgroups are the control limits considered reliable estimates?

Answer: 20–25 subgroups

A minimum of 20–25 subgroups is generally recommended to establish stable and reliable control limits.

In a p-chart, what does the plotted statistic represent?

Answer: The proportion of defective items in a subgroup

The p-chart plots the proportion (fraction) of nonconforming items in each subgroup.

Which measure of central tendency is plotted on the center line of a standard X-bar chart?

Answer: The grand average of subgroup means

The center line of an X-bar chart is the grand mean (X-double-bar), calculated as the average of all subgroup averages.

A cause-and-effect (Ishikawa) diagram is most useful for:

Answer: Brainstorming potential root causes of a quality problem

The fishbone diagram organizes potential causes into categories (e.g., Man, Machine, Method, Material) to systematically identify root causes.

What is 'sigma estimator d2' used for in SPC?

Answer: Estimating process standard deviation from subgroup ranges

The d2 constant is used to convert the average subgroup range (R-bar) into an unbiased estimate of the process standard deviation.