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MACHART Statement |
See MACMA2 in the SAS/QC Sample Library |
By default,
the MACHART statement estimates the process mean ()and standard deviation (
)from the data. This is illustrated
in the "Getting Started"
section of this chapter. 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 metal clip manufacturing
process (introduced
in "Creating Moving Average Charts from Raw Data" ) has a mean of
15 and standard deviation of 0.2. The following
statements specify these standard values:
title 'Specifying Standard Process Mean and Standard Deviation'; symbol v=dot c=yellow; proc macontrol data=clips1; machart gap*day / mu0 = 15 sigma0 = 0.2 span = 4 xsymbol = mu0 cframe = steel cinfill = vpab connect = yellow coutfill = salmon; run;
The XSYMBOL= option specifies the label for the central line. The resulting chart is shown in Output 21.1.1.
Output 21.1.1: Specifying Standard Values with MU0= and SIGMA0=
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The central line and control limits
are determined using and
(see
the equations in Table 21.19).
Output 21.1.1 indicates that the process is out-of-control since
the moving averages for DAY=17, DAY=19, and DAY=20 lie below the lower
control limit.
You can also specify and
with the
variables _MEAN_ and _STDDEV_ in a LIMITS= data set,
as illustrated by the following statements:
data cliplim; length _var_ _subgrp_ _type_ $8; _var_ = 'gap'; _subgrp_ = 'day'; _type_ = 'STANDARD'; _limitn_ = 5; _mean_ = 15; _stddev_ = 0.2; _span_ = 4; run; proc macontrol data=clips1 limits=cliplim; machart gap*day / xsymbol=mu0; run;
The variable _SPAN_ is required, and its value provides the number of terms in the moving average. The variables _VAR_ and _SUBGRP_ are also required, and their values must match the process and subgroup-variable, respectively, specified in the MACHART 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 chart (not shown here) is identical to the one shown in Output 21.1.1.
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