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IRCHART Statement |

See SHWIREX2 in the SAS/QC Sample Library |

By default, the IRCHART statement estimates the process mean () and standard deviation ()from the data, as in the previous example. However, there are applications in which known (standard) values and are available for these parameters based on previous experience or extensive sampling.

For example, suppose that the manufacturing process
described in the previous example produces engines whose weights
are normally distributed with a mean of
1250 and a standard deviation of 12. The following
statements create individual measurements and moving range
charts based on these values:

title 'Specifying Standard Process Mean and Standard Deviation'; symbol v=dot c=yellow; proc shewhart data=engines; irchart weight*id / mu0 = 1250 sigma0 = 12 xsymbol = mu0 cframe = vigb cinfill = vlib cconnect = yellow coutfill = red; run;

The charts are shown in Output 34.2.1. The MU0= option and SIGMA0= option specify and .The XSYMBOL= option specifies the label for the central line on the individual measurements chart, and the keyword MU0 requests a label indicating that the central line is based on a standard value.

You can also specify and as the values of the variables _MEAN_ and _STDDEV_ in a LIMITS= data set. For example, the following statements create a LIMITS= data set with the standard values specified in the preceding IRCHART statement:

data englim; length _var_ _subgrp_ _type_ $8; _var_ = 'weight'; _subgrp_ = 'id'; _limitn_ = 2; _type_ = 'STANDARD'; _mean_ = 1250; _stddev_ = 12; run;

The variables _VAR_ and _SUBGRP_ are required, and their
values must
match the *process* and *subgroup-variable*, respectively,
specified in the IRCHART statement.
The bookkeeping variable _TYPE_ is not required,
but it is recommended to
indicate that the variables _MEAN_ and _STDDEV_ provide
standard values rather than estimated values.
See "LIMITS= Data Set" for details.

The following statements read ENGLIM as a LIMITS= data set:

proc shewhart data=engines limits=englim; irchart weight*id / xsymbol=mu0; run;

The resulting charts (not shown here) are identical to those shown in Output 34.2.1.

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