Quality Control

Acceptance Sampling

The process of examining a portion of received goods rather than the entire lot or shipment.

Sampling Plan

The determination of the sample size and number of defectives that will trigger rejection of a lot.

Producer's Risk

The probability that a good lot with acceptable quality will be rejected by the consumer.

Consumer's Risk

The probability that a bad lot with unacceptable quality will be accepted by the consumer.

Type I Error

An event where the consumer rejects a good lot (that was erroneously identified as bad through the acceptance sampling process).

Type II Error

An event where the consumer accepts a bad lot (that was erroneously identified as good through the acceptance sampling process).

Statistical Process Control

The application of quantitative methods to monitor and classify the variability within a process (natural-cause or assignable-cause variation).

Natural Causes

The source of expected and in-control variation within properly functioning process.

Assignable Causes

The source of unexpected out-of-control variation within a process.

Out of Control Condition

The status of a process when sample data does not fall within control limits.

Variables Control Charts

SPC charts constructed from continuous measurable data (weight, height, length, time, etc.).

Attributes Control Charts

SPC charts constructed from discrete countable data (pass/fail, yes/no, etc.).

Process Capability

The measurement of how well a process performs against design specifications.

Process Capability Ratio

A way to determine if the variability of an in-control process will meet design specifications.

Process Capability Index

A way to determine if an in-control process is properly aligned or centered with design specifications.

Control Limit

The upper or lower boundary within a control chart that separates natural-cause variation from assignable-cause variation.

x-Bar Chart

A run chart that evaluates the stability of process sample averages.

R-Chart

A run chart that evaluates the stability of process sample dispersion.

p-Chart

A run chart that monitors the number of defective products within a group of identical finished products.

c-Chart

A run chart that monitors the number of flaws within one unit of production or service.