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MACHART Statement |

See MACMA1 in the SAS/QC Sample Library |

In the manufacture of a metal clip, the gap between the ends of the clip is a critical dimension. To monitor the process for a change in the average gap, subgroup samples of five clips are selected daily. The data are analyzed with a uniformly weighted moving average chart. The gaps recorded during the first twenty days are saved in a SAS data set named CLIPS1.

data clips1; input day @ ; do i=1 to 5; input gap @ ; output; end; drop i; datalines; 1 14.76 14.82 14.88 14.83 15.23 2 14.95 14.91 15.09 14.99 15.13 3 14.50 15.05 15.09 14.72 14.97 4 14.91 14.87 15.46 15.01 14.99 5 14.73 15.36 14.87 14.91 15.25 6 15.09 15.19 15.07 15.30 14.98 7 15.34 15.39 14.82 15.32 15.23 8 14.80 14.94 15.15 14.69 14.93 9 14.67 15.08 14.88 15.14 14.78 10 15.27 14.61 15.00 14.84 14.94 11 15.34 14.84 15.32 14.81 15.17 12 14.84 15.00 15.13 14.68 14.91 13 15.40 15.03 15.05 15.03 15.18 14 14.50 14.77 15.22 14.70 14.80 15 14.81 15.01 14.65 15.13 15.12 16 14.82 15.01 14.82 14.83 15.00 17 14.89 14.90 14.60 14.40 14.88 18 14.90 15.29 15.14 15.20 14.70 19 14.77 14.60 14.45 14.78 14.91 20 14.80 14.58 14.69 15.02 14.85 ;

The following statements produce the listing of the
data set CLIPS1 shown in Figure 21.1:

title 'The Data Set CLIPS1'; proc print data=clips1 noobs; run;

The data set CLIPS1
is said to be in "strung-out" form, since
each observation contains the day and gap measurement
of a single clip.
The first five observations contain
the gap measurements for the first day, the second five
observations contain the gap measurements for the second
day, and so on. Because the variable DAY classifies the
observations into rational subgroups, it is referred to
as the *subgroup-variable*. The variable GAP contains
the gap measurements and is referred to as the
*process variable* (or *process* for short).

The within-subgroup variability of the gap measurements is known to be stable. You can use a uniformly weighted moving average chart to determine whether the mean level is in control. The following statements create the chart shown in Figure 21.2:

title 'Moving Average Chart for Gap Measurements'; symbol v=dot c=yellow; proc macontrol data=clips1; machart gap*day / span=3 cframe = steel cinfill = vpab cconnect = yellow coutfill = salmon; run;

This example illustrates the basic form of the MACHART
statement. After the keyword MACHART, you
specify the *process*
to analyze (in this case, GAP) followed by an asterisk and
the *subgroup-variable* (DAY).
The SPAN= option specifies the number of
terms to include in the moving average.
Options such as SPAN= are specified after the slash (/)
in the MACHART statement. A complete list of options
is presented in the "Syntax" section.
You must provide the span of the moving average.
As an alternative
to specifying the SPAN=
option, you can read the span from an input
data set; see "Reading Preestablished Control Limit Parameters"
.

The input data set is specified with the DATA= option in the PROC MACONTROL statement.

Each point on the chart represents the uniformly weighted
moving average
for a particular day.
The moving average *A _{1}*
plotted at DAY=1 is simply the subgroup mean for DAY=1.
The moving average

For succeeding days, the moving average is similarly calculated as the average of the present and the two previous subgroup means (since a span of three is specified with the SPAN= option).

Note that the moving average for the seventh day lies above the upper control limit, signaling an out-of-control process.

By default, the control limits shown are limits estimated from the data; the formulas for the limits are given in Table 21.19.

For computational details, see "Constructing Uniformly Weighted Moving Average Charts" . For more details on reading from a DATA= data set, see "DATA= Data Set" .

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