Plots c or u charts representing numbers of defective items (A.F. Kane & R.W. Payne).
|What to print (
||Type of chart to plot (
||Method to use to obtain the control limits (
||Multiplier to use to test whether to use mean sample size for control limits; default 1|
||Which high-resolution graphics window to use; default 3|
||Whether or not to clear the graphics screen before plotting (
||Number of defective items|
||Number of items tested|
||Sets or saves centre line|
||Sets or saves lower control limit|
||Sets or saves upper control limit|
The c and u charts are used in statistical process control to evaluate testing schemes in which numbers of defects are measured in successive batches of items. The number of defects per batch is specified, in a variate, by the
NDEFECTIVE parameter. The
NTESTED parameter supplies the number of items in each batch – this can be a scalar if the batches are all of the same size, otherwise it is a variate.
PLOT option controls the type of chart: the c chart plots number of defects per batch, while the u chart plots the number of defects per item.
The charts contain not only the observed numbers of defects but also a centre line (indicating a target value) and lines showing upper and lower control limits (bounding the zone outside which the process is said to be out of control). The control limits relevant to each batch in a u chart will depend on the batch sizes. The
TOLERANCE option determines whether an average sample size is used if the individual sizes are not exactly equal: this will happen unless either
MIN(NTESTED) * TOLERANCE < MEAN(TESTED)
MEAN(TESTED) * TOLERANCE < MAX(NTESTED)
METHOD option specifies how the various lines are to be defined, with the following settings.
||this is the default setting, and requests the method conventionally used in SPC. For a c chart, the centre line is at|
|c = (total number defects) / (number of batches)|
|and the limits are at c + 3 × √(c). For a u chart, the centre line is at|
|u = (total number defects) / (total number of items)|
|and the limits are at u + 3 × √(u/n).|
||specifies that the values are supplied by the
||obtains the values by fitting a generalized linear model with Poisson distribution and log link.|
For settings of
METHOD other than
UPPERCONTROLLIMIT parameters can be used to save the centre line and limits.
You can set
PRINT=warnings to list any batches that are outside the control limits; by default these are suppressed. As usual, the
WINDOW option specifies which high-resolution graphics window to use for the plot, and the
SCREEN option controls whether or not to clear the graphics screen before plotting.
For further information about the standard SPC methods see for example Chapter 5 of Montgomery (1985). Section 3.5 of the Guide to the Genstat Command Language, Part 2 Statistics gives more details about generalized linear models.
Any restrictions are ignored.
Montgomery, D.C. (1985). Introduction to Statistical Process Control. Wiley, New York.
Commands for: Six sigma.
CAPTION 'SPCCHART example',\ !t('1) Data from Montgomery (1985), Introduction',\ 'to Statistical Process Control, page 174.'); STYLE=meta,plain VARIATE [VALUES=21,24,16,12,15,5,28,20,31,25,20,24,16,\ 19,10,17,13,22,18,39,30,24,16,19,17,15] Nonconformities SPCCHART [PRINT=warnings] Nonconformities; NTESTED=100 CAPTION !t('2) Data from Montgomery (1985, Introduction',\ 'to Statistical Process Control, page 181.)') VARIATE [VALUES=10,12,8,14,10,16,11,7,10,15,9,5,7,11,12,6,8,10,7,5]\ Nonconformities SPCCHART [PRINT=warnings; PLOT=u] Nonconformities; NTESTED=5