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XRCHART Statement |

See SHWXR4 in the SAS/QC Sample Library |

The following data set (WIRE) contains breaking strength measurements recorded in pounds per inch for 25 samples from a metal wire manufacturing process. The subgroup sample sizes vary between 3 and 7.

data wire; input day size @; informat day date7.; format day date7.; do i=1 to size; input brstr @@; output; end; drop i size; label brstr ='Breaking Strength (lb/in)'; datalines; 20JUN94 5 60.6 62.3 62.0 60.4 59.9 21JUN94 5 61.9 62.1 60.6 58.9 65.3 22JUN94 4 57.8 60.5 60.1 57.7 23JUN94 5 56.8 62.5 60.1 62.9 58.9 24JUN94 5 63.0 60.7 57.2 61.0 53.5 25JUN94 7 58.7 60.1 59.7 60.1 59.1 57.3 60.9 26JUN94 5 59.3 61.7 59.1 58.1 60.3 27JUN94 5 61.3 58.5 57.8 61.0 58.6 28JUN94 6 59.5 58.3 57.5 59.4 61.5 59.6 29JUN94 5 61.7 60.7 57.2 56.5 61.5 30JUN94 3 63.9 61.6 60.9 01JUL94 5 58.7 61.4 62.4 57.3 60.5 02JUL94 5 56.8 58.5 55.7 63.0 62.7 03JUL94 5 62.1 60.6 62.1 58.7 58.3 04JUL94 5 59.1 60.4 60.4 59.0 64.1 05JUL94 5 59.9 58.8 59.2 63.0 64.9 06JUL94 6 58.8 62.4 59.4 57.1 61.2 58.6 07JUL94 5 60.3 58.7 60.5 58.6 56.2 08JUL94 5 59.2 59.8 59.7 59.3 60.0 09JUL94 5 62.3 56.0 57.0 61.8 58.8 10JUL94 4 60.5 62.0 61.4 57.7 11JUL94 4 59.3 62.4 60.4 60.0 12JUL94 5 62.4 61.3 60.5 57.7 60.2 13JUL94 5 61.2 55.5 60.2 60.4 62.4 14JUL94 5 59.0 66.1 57.7 58.5 58.9 ;

The following statements request and *R* charts, shown in
Output 43.3.1, for the strength measurements:

title 'Mean and Range Charts for Breaking Strengths'; symbol v=dot c=salmon; proc shewhart data=wire; xrchart brstr*day / nohlabel cframe = bigb cinfill = ywh cconnect = salmon; run;

Note that the central line on the

The XRCHART statement provides various options for working with unequal subgroup sample sizes. For example, you can use the LIMITN= option to specify a fixed (nominal) sample size for computing control limits, as illustrated by the following statements:

proc shewhart data=wire; xrchart brstr*day / nohlabel limitn = 5 cframe = bigb cinfill = ywh cconnect = salmon; run;

The resulting charts are shown in Output 43.3.2.

Note that the only points displayed on the chart are those corresponding to subgroups whose sample sizes match the nominal sample size of five. To plot points for all subgroups (regardless of subgroup sample size), you can specify the ALLN option, as follows:

proc shewhart data=wire; xrchart brstr*day / nohlabel limitn = 5 alln nmarkers cframe = bigb cinfill = ywh cconnect = salmon; run;

The charts are shown in Output 43.3.3. The NMARKERS option requests special symbols to identify points for which the subgroup sample size differs from the nominal sample size.

You can use the SMETHOD= option to determine how the process standard deviation is to be estimated when the subgroup sample sizes vary. The default method computes as an unweighted average of subgroup estimates of .Specifying SMETHOD=MVLUE requests an estimate that assigns greater weight to estimates of from subgroups with larger sample sizes. For more information, see "Methods for Estimating the Standard Deviation" .

The following statements apply both methods:

proc shewhart data=wire; xrchart brstr*day / outlimits = wlim1 outindex = 'Default' nochart; xrchart brstr*day / smethod = mvlue outlimits = wlim2 outindex = 'MVLUE' nochart; run; data wlimits; set wlim1 wlim2; run;

The data set WLIMITS is listed in Output 43.3.4.

The variables in an OUTLIMITS= data set whose values vary with subgroup sample size are assigned the special missing value

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