Six Sigma Statistical Terms
UNNATURAL PATTERN
Any pattern in which a significant number of the measurements do not group themselves around a central tendency. When the pattern is unnatural, it means that non-random disturbances are present and are affecting the process.
UPPER CONTROL LIMIT
A horizontal line on a control chart (usually dotted) which represents the upper limits of capability for a process operating with only random variation. VARIABLE A characteristic that may take on different values.
VARIABLES DATA
Data collected from a process input or output where the measurement scale has a significant level of subdivisions or resolution, e.g., ohms, voltage, diameter, etc. VARIATION Any quantifiable difference between individual measurements; such differences can be classified as being due to common causes (random) or special causes (assignable).
VARIATION RESEARCH
Procedures, techniques, and methods used to isolate one type of variation from another (for example, separating product variation from test variation).
X & R CHARTS
A control chart which is a representation of process capability over time; displays the variability in the process average and range across time.
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