#### Example 17.39 Specifying Standard Values for the Process Mean and Standard Deviation

See SHWXR3 in the SAS/QC Sample LibraryBy default, the XRCHART statement estimates the process mean () and standard deviation () from the data, as in the previous example. However, there are applications in which standard values ( and ) are available based, for instance, on previous experience or extensive sampling. You can specify these values with the MU0=
and SIGMA0= options.

For example, suppose it is known that the adhesive coating process introduced in the previous example has a mean of 1260 and
standard deviation of 15. The following statements specify these standard values:

ods graphics off;
title 'Specifying Standard Process Mean and Standard Deviation';
proc shewhart history=Tape;
xrchart Weight*Sample / mu0 = 1260
sigma0 = 15
xsymbol = mu0;
run;

The XSYMBOL= option specifies the label for the central line on the chart. The resulting and R charts are shown in Output 17.39.1.

Output 17.39.1: Specifying Standard Values with MU0= and SIGMA0=

The central lines and control limits for both charts are determined by using and (see the equations in Table 17.72). Output 17.39.1 indicates that the process is in statistical control.

You can also specify and with the variables `_MEAN_`

and `_STDDEV_`

in a LIMITS= data set, as illustrated by the following statements:

data Tapelim;
length _var_ _subgrp_ _type_ $8;
_var_ = 'Weight';
_subgrp_ = 'Sample';
_type_ = 'STANDARD';
_limitn_ = 5;
_mean_ = 1260;
_stddev_ = 15;
proc shewhart history=Tape limits=Tapelim;
xrchart Weight*Sample / xsymbol=mu0;
run;

The variables `_VAR_`

and `_SUBGRP_`

are required, and their values must match the *process* and *subgroup-variable*, respectively, specified in the XRCHART 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.

The resulting charts (not shown here) are identical to those shown in Output 17.39.1.