The SHEWHART Procedure |
The following notation is used in this section:
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process mean (expected value of the population of measurements) |
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process standard deviation (standard deviation of the population of measurements) |
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mean of measurements in th subgroup |
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sample size of th subgroup |
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the number of subgroups |
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th measurement in the th subgroup, |
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th largest measurement in the th subgroup: |
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weighted average of subgroup means |
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median of the measurements in the th subgroup: |
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average of the subgroup medians: |
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median of the subgroup medians. Denote the th largest median by so that . |
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standard error of the median of independent, normally distributed variables with unit standard deviation (the value of can be calculated with the STDMED function in a DATA step) |
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th percentile of the distribution of the median of independent observations from a normal population with unit standard deviation |
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th percentile of the standard normal distribution |
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th percentile of the distribution of the range of independent observations from a normal population with unit standard deviation |
A box-and-whisker plot is displayed for the measurements in each subgroup on the box chart. Figure 13.14 illustrates the elements of each plot.
The skeletal style of the box-and-whisker plot shown in Figure 13.14 is the default. You can specify alternative styles with the BOXSTYLE= option; see Example 13.2 or the entry for BOXSTYLE= in Dictionary of Options
You can compute the limits in the following ways:
as a specified multiple () of the standard error of (or ) above and below the central line. The default limits are computed with (these are referred to as limits).
as probability limits defined in terms of , a specified probability that (or ) exceeds the limits
The CONTROLSTAT= option specifies whether control limits are computed for subgroup means (the default) or subgroup medians. The following tables provide the formulas for the limits:
CONTROLSTAT=MEAN |
CONTROLSTAT=MEDIAN |
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LCLX lower limit = |
LCLM lower limit = |
Central Line = |
Central Line = |
UCLX upper limit = |
UCLM upper limit = |
CONTROLSTAT=MEAN |
CONTROLSTAT=MEDIAN |
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LCLX lower limit = |
LCLM lower limit = |
Central Line = |
Central Line = |
UCLX upper limit = |
UCLM upper limit = |
In the preceding tables, replace with if you specify MEDCENTRAL=AVGMEAN in addition to CONTROLSTAT=MEDIAN. Likewise, replace with if you specify MEDCENTRAL=MEDMED in addition to CONTROLSTAT=MEDIAN. If standard values and are available for and , replace with and with in Table 13.5 and Table 13.6.
Note that the limits vary with . The formulas for median limits assume that the data are normally distributed.
You can specify parameters for the limits as follows:
Specify with the SIGMAS= option or with the variable _SIGMAS_ in a LIMITS= data set.
Specify with the ALPHA= option or with the variable _ALPHA_ in a LIMITS= data set.
Specify a constant nominal sample size for the control limits with the LIMITN= option or with the variable _LIMITN_ in a LIMITS= data set.
Specify with the MU0= option or with the variable _MEAN_ in a LIMITS= data set.
Specify with the SIGMA0= option or with the variable _STDDEV_ in a LIMITS= data set.
Note:You can suppress the display of the control limits with the NOLIMITS option. This is useful for creating standard side-by-side box-and-whisker plots.
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