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Alpha risk: Pertains to sampling and the potential risk that bad
product will be accepted and shipped to the consumer. Known as
consumers risk.
Average run lengths (ARL): On a control chart, the number of
subgroups expected to be inspected before a shift in magnitude takes
place.
Beta risk: Pertains to sampling and potential risk that good product
will be rejected and not shipped to the consumer. Known as
producers risk.
Central limit theorem: The central limit theorem enables conclusions
to be drawn from sample data and applied to population. The
theorem states that a group of sample averages tend to be normally
distributed, regardless of the shape of the originating population. As
the sample size increases above four, the tendency toward normality
improves. The mean of the sample averages will approximate the
mean of the population. The standard deviation of the sample
averages is related to the standard deviation of the population by the
square root of the sample size.
Std. Dev.(samples) = ((Std. Dev. (Population)/square root of sample
size)
Common causes: Causes of variation that are inherent in a process
over time. They affect every outcome of the process and everyone
working in the process (see also "special causes").
Control charts: A chart with upper and lower control limits on which
values of some statistical measure for a series of samples or
subgroups are plotted. The chart frequently shows a central line to
help detect a trend of plotted values toward either control limit.
Control limits: The natural boundaries of a process within specified
confidence levels, expressed as the upper control limit (UCL) and the
lower control limit (LCL).
Cp index: The ratio of tolerance to six sigma, or the USL (upper
specification limit) minus the LSL (lower specification limit) divided by
six sigma. It is sometimes referred to as the engineering tolerance
divided by the natural tolerance and is only a measure of dispersion.
Cpk index: Equals the lesser of the USL minus the mean divided by
three sigma (or the mean) minus the LSL divided by three sigma. The
greater the Cpk value, the better.
Evidence of assignable cause:
Process capability: A statistical measure of the inherent process
variability for a given characteristic. The most widely accepted
formula for process capability is six sigma.
Process capability index: The value of the tolerance specified for
the characteristic divided by the process capability. The several
types of process capability indexes include the widely used Cpk and
Cp .
Rational subgroup: A small sample selected from a process in such
a way as to minimize the variation within the subgroup and allow for
variation between the subgroup. Basis for control charts.
Run chart: A chart showing a line connecting numerous data points
collected from a process running over a period of time.
Special causes: Causes of variation that arise because of special
circumstances. They are not an inherent part of a process. Special
causes are also referred to as assignable causes (see also "common
causes").
Specification: A document that states the requirements to which a
given product or service must conform. ]
Specification limits: The boundaries of a process within specified
levels, expressed as the upper specification limit (USL) and the
lower specification limit (LSL).
Statistical control: A state of statistical control exists when the
process is stable and is being acted upon by only common causes.
Tolerance: The maximum and minimum limit values a product may
have and still meet customer requirements.
Variation, categories:
Within piece
Between pieces
Between processes
Between times
Measurement
Variation, sources
Equipment
Material
Environment
Operator
Inspection