BS ISO 22514-4:2016 pdf download

BS ISO 22514-4:2016 pdf download.Statistical methods in process management — Capability and performance — Part 4: Process capability estimates and performance measures
3 Basic concepts used for process capability and performance
3.1 General
The measures referred to in 4.2 to 4.6 refer only to measured data. They are unsuitable for count or attributes data and information concerning the expression of measures for such data will be found in ISO 22514-5.
3.2 Location
The characterization of location is the mean, μ, or the median, X 50 % . Although for symmetric distributions the mean is the most natural selection, with non-symmetric distributions the median is the preferred selection.
3.3 Dispersion
3.3.1 Inherent dispersion
The preferred selection to quantify inherent dispersion is the standard deviation σ. This is often estimated from the mean range value, R , taken from a range (R) chart or S from a standard deviation (S) chart when the process is stable and in a state of statistical control as indicated in 4.1. Methods used to estimate the process standard deviation are given in Annex A.
3.3.2 Total dispersion
It is necessary to differentiate between a standard deviation that measures only short-term variation and that which measures longer-term variation. The total dispersion is the dispersion that is inherent in the long-term variation. Methods of calculating the standard deviations representing these variations are given in Annex A. Very often, when data are gathered over a long period of time, the standard deviation is made larger by the effects of fluctuations in the process, σ t .
3.3.3 Short-term dispersion
A process may have a short-term dispersion effect that is a part of the total dispersion. Figure 1 illustrates this. The short-term dispersion includes the inherent dispersion and can also include some short-term instability effect.
3.4 Mean square error (MSE)
When minimizing variation, some practitioners use the mean square error as the preferred measure. It is compatible with the methods used in off-line quality techniques.
3.5 Reference limits
The lower and upper reference limits are respectively defined as the 0,135 % and the 99,865 % percentiles of the distribution that describe the output of the process characteristic. They are written as X 0,135 5 % and X 99,865 % .
3.6 Reference interval (also known as process spread)
The reference interval is the interval between the upper and the lower reference limits. The reference interval includes 99,73 % of the individuals in the population from a process that is in a state of statistical control.
4.1 General
Process capability is a measure of inherent process variability. The variability that is inherent in a process when operating in a state of statistical control is known as the inherent process variability. It represents the variation that remains after all known removable assignable causes have been eliminated. If the process is monitored using a control chart, the control chart will show an in control state. Capability is often regarded as being related to the proportion of output that will occur within the product specification tolerances. Since a process in statistical control should be described by a predictable distribution, the proportion of out-of-specification outputs can be estimated. As long as the process remains in statistical control, it will continue to produce the same proportion out-of- specification.