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Control limit. A line (or lines) on a control chart used as a basis for judging the significance of the variation from chart point to chart point. Variation beyond a control limit is evidence that assignable causes are affecting the process.
Control limits are calculated from process data and are not to be confused with engineering or administrative specifications.
A control chart usually displays a lower control limit (LCL) and an upper control limit (UCL).
Control limit.
Used in a control chart as the basis for measuring the point-to-point variation.
Variation beyond a control limit proves that certain causes will affect the process.
Control limits are calculated from process data; do not confuse them with specification limits.
A control chart usually displays a lower control limit (LCL) and an upper control limit (UCL).
Note: in general, the UCL and LCL are positioned at plus or minus three standard deviations from the mean.
 

Process Accuracy: Ca
The deviation of the process mean from the midpoint of the specification limits.
Measures the consistency between the actual process mean and the specification center value; the degree of deviation between the actual mean obtained from production product data and the specification standard value.
 

Process Precision: Cp / Capability of process.
The greater the number, the better the process is capable of meeting the specification limits.
A process is judged to be capable when CP is at least 1.0.
Measures how the variation width of the process compares with the specification tolerance range.
In general, CP must be greater than one before mass production can begin; otherwise the cost arising from variation will be too high.
 

Process Capability Index: Cpk
Process Capability index related to both dispersion and centeredness.
A process capability indicator concerning dispersion and centeredness.
A comprehensive process capability indicator that considers both the variation width and the process standard value.
An index that combines the two values of Ca and Cp.
 

CpL
Distance between the process mean and the lower specification limit scaled using the capability sigma.
The gap value between the process standard value and the lower specification limit.
 

CpM
Process capability index that is affected by the difference between the sample mean and the target value.
A process capability indicator affected by the difference between the sample center value and the specification center value.
 

CpU
Distance between the process mean and the upper specification limit scaled using the capability sigma.
The gap between the process standard value and the upper specification limit; usually used for one-sided specifications.
 

CR
The reciprocal of CP. The smaller the number, the better the process is capable of meeting the specification limits.
CR*100% is the percentage of the specification band used up by the process.
The reciprocal of CP. The smaller this number, the better the process meets the specification limits. The CR percentage represents the specification width consumed by the process.
 

Pp
Most commonly used process performance index.
This is a more reliable measure of process performance for folded normal distribution since folded normal distribution is one-sided.
This is a more reliable measure of process capability for the folded normal distribution, because the folded normal distribution is one-sided.
 

PpK
Process performance index related to both dispersion and centeredness.
This index is not suitable for folded normal distribution, which is one-sided.
A comprehensive process capability indicator that considers both the variation width and the process standard value.
However, this indicator is not suitable for the folded normal distribution, because it is one-sided.
 

PpL
Distance between the process mean and the lower specification limit scaled using the process standard deviation.
This index is not suitable for folded normal distribution, which is one-sided.
The gap value between the process standard value and the lower specification limit.
However, this indicator is not suitable for the folded normal distribution, because it is one-sided.
 

Ppm
Process performance index that is affected by the difference between the sample mean and the target value.
A process capability indicator affected by the difference between the sample center value and the specification center value
 

PpU
Distance between the process mean and the upper specification limit scaled using the process standard deviation.
This index is not suitable for folded normal distribution, which is one-sided.
The gap between the process standard value and the upper specification limit. However, this indicator is not suitable for the folded normal distribution, because it is one-sided.
 

PR
PR. Reciprocal of PP.
The reciprocal of PP
 

Kurtosis
A measure of peakedness.
The population kurtosis is given by the fourth moment over the square of the variance.
This ratio equals 0 for any normal distribution.
A positive ratio indicates an excess of values in the neighborhood of the mean
(with a depletion of the “flanks” of the curve representing the distribution).
and a negative ratio corresponds to a curve with a flatter top than the normal curve has.
A measure of peakedness.
The most commonly used kurtosis is four times the moment multiplied by the square of the variance.
For any normal distribution, this ratio is 0.
A positive probability denotes values near the mean that exceed it, while a negative probability denotes a curve that exceeds the normal one.
 

Skewness
A measure of departure from normality.
A distribution is skewed if it isn’t symmetric but has more cases (a “tail”) toward one end of the distribution than the other.
If the tail is toward larger values, the distribution is positively skewed, or skewed to the right.
If the tail is toward smaller values, the distribution is negatively skewed, or skewed to the left.
A measure of the departure from the normal distribution.
If a distribution is not symmetric, it skews toward one side and presents a single tail.
If it skews toward larger values, the distribution shows positive skewness; otherwise it shows negative skewness.
 

Interquartile range, IQR
The interquartile range is, among the first quartile Q1, the second quartile Q2, and the third quartile Q3, the largest value Q3 minus the smallest value Q1, that is, Q3 – Q1.