Note: See X-bar and R CHARTS-Specifying Standard Values in the SAS/QC Sample Library.
By 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 on; title 'Specifying Standard Process Mean and Standard Deviation'; proc shewhart history=Tape; xrchart Weight*Sample / mu0 = 1260 sigma0 = 15 xsymbol = mu0 odstitle = title; 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.
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.