SPC Glossary 

Your quick reference to statistical process control for manufacturing quality managment systems.

Quality Management System Glossary: T-Z



Any member of a family of continuous probability distributions that arises when estimating the mean of a normally distributed population in situations where the sample size is small, and population standard deviation is unknown.


An action taken to compensate for variation within the control limits of a stable system. Tampering increases (rather than decreases) variation, as in the case of Over Control.


The maximum and minimum limit values a product can have and still meet customer requirements.


The graphical representation of a variable’s tendency, over time, to increase, decrease, or remain unchanged.

Trend Control Chart

A control chart in which the deviation of the subgroup average, X-bar, from an expected trend in the process level is used to evaluate the stability of a process.

Type I Error

An incorrect decision to reject something (such as a statistical hypothesis or a lot of products) when it is acceptable.

Type II Error

An incorrect decision to accept something when it is unacceptable.



Count-per-unit chart.


An object for which a measurement or observation can be made; commonly used in the sense of a unit of product or piece, the entity of product inspected to determine whether it is defective or non-defective.

Upper Control Limit (UCL)

Control limit for points above the central line in a control chart.


Variable Data

Measurement information. Control charts based on variable data include average (X-bar) chart, range (R) chart, and sample standard deviation (or s) chart.


In probability theory and statistics, variance is the expectation of the squared deviation of a random variable from its mean. Informally, it measures how far a set of (random) numbers are spread out from their average value.


A change in data, characteristic or function caused by one of four factors: special causes, common causes, tampering, or structural variation.


Weibull Distribution

Named after Swedish mathematician Waloddi Weibull, the Weibull Distribution is a continuous probability distribution. Commonly used to assess product reliability, analyze life data, and model failure times. 



A control chart used for process in which individual measurements of the process are plotted for analysis. Also called an Individuals chart or I-chart.

X-Bar Chart

A control chart used for processes in which the averages of subgroups of process data are plotted for analysis.


Zero Defects

A management tool aimed at the reduction of defects through prevention. Directed at motivating people to prevent mistakes by developing a constant, conscious desire to do their job right the first time. Developed by quality expert Philip B. Crosby.

Z1.4 and Z1.9

ANSI/ASQ Z1.4-2003 (R2013): Sampling Procedures and Tables for Inspection by Attributes is an acceptance sampling system to be used with switching rules on a continuing stream of lots for the acceptance quality limit (AQL) specified.

ANSI/ASQ Z1.9-2003 (R2013): Sampling Procedures and Tables for Inspection by Variables for Percent Nonconforming is an acceptance sampling system to be used on a continuing stream of lots for the AQL specified.

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