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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range 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. Then |
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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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100th percentile of the distribution of the median of independent observations from a normal population with unit standard deviation |
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expected value of the range of independent normally distributed variables with unit standard deviation |
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standard error of the range of independent observations from a normal population with unit standard deviation |
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100th percentile of the standard normal distribution |
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100th percentile of the distribution of the range of independent observations from a normal population with unit standard deviation |
Each point on a median chart indicates the value of a subgroup median (). For example, if the tenth subgroup contains the values 12, 15, 19, 16, and 14, the value plotted for this subgroup is . Each point on a range chart indicates the value of a subgroup range (). For example, the value plotted for the tenth subgroup is .
On a median chart, the value of the central line indicates an estimate for , which is computed as
by default
when you specify MEDCENTRAL=AVGMEAN
when you specify MEDCENTRAL=MEDMED
when you specify with the MU0= option
On the range chart, by default, the central line for the th subgroup indicates an estimate for the expected value of , which is computed as , where is an estimate of . If you specify a known value () for , the central line indicates the value of . The central line on the range chart varies with .
You can compute the limits
as a specified multiple () of the standard errors of and 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 its limits
The following table provides the formulas for the limits:
Control Limits |
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Median Chart |
LCL lower limit = |
UCL upper limit = |
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Range Chart |
LCL lower control limit = |
UCL upper control limit = |
Probability Limits |
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Median Chart |
LCL lower limit = |
UCL upper limit = |
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Range Chart |
LCL lower limit = |
UCL upper limit = |
In Table 13.32, replace with if you specify MEDCENTRAL=AVGMEAN, and replace with if you specify MEDCENTRAL=MEDMED. Replace with if you specify with the MU0= option, and replace with if you specify with the SIGMA0= option.
The formulas assume that the data are normally distributed. Note that the limits for both charts vary with and that the probability limits for are asymmetric around the central line.
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 the LIMITS= data set.
Specify with the SIGMA0= option or with the variable _STDDEV_ in the LIMITS= data set.
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