T
T-Distribution
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.
Tampering
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.
Tolerance
The maximum and minimum limit values a product can have and still meet customer requirements.
Trend
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.
U
U-Chart
Count-per-unit chart.
Unit
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.
V
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.
Variance
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.
Variation
A change in data, characteristic or function caused by one of four factors: special causes, common causes, tampering, or structural variation.
W
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.
X
X-Chart
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.
Z
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.