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UCHART Statement 
See SHWUEX3 in the SAS/QC Sample Library 
In the fabric
manufacturing process described in "Creating u Charts from Defect Count Data",
each roll of fabric is
30 meters long, and an inspection unit is defined as one
square meter.
Thus, there are 30 inspection units in each subgroup sample.
Suppose now that the length of each piece of fabric varies.
The following statements create a SAS data set (FABRICS2)
that contains the number of fabric
defects and size (in square meters) of 25 pieces of fabric:
data fabrics2; input roll defects sqmeters @@; datalines; 1 7 30.0 2 11 27.6 3 15 30.4 4 6 34.8 5 11 26.0 6 15 28.6 7 5 28.0 8 10 30.2 9 8 28.2 10 3 31.4 11 3 30.3 12 14 27.8 13 3 27.0 14 9 30.0 15 7 32.1 16 6 34.8 17 7 26.5 18 5 30.0 19 14 31.3 20 13 31.6 21 11 29.4 22 6 28.6 23 6 27.5 24 9 32.6 25 11 31.7 ;
A listing of FABRICS2 is shown in Output 41.3.1.
Output 41.3.1: The Data Set FABRICS2The following statements request a u chart for the number of
defects per square meter:
title 'u Chart for Fabric Defects per Square Meter'; symbol v=dot c=vig; proc shewhart data=fabrics2; uchart defects*roll / subgroupn = sqmeters outlimits = flimits cframe = steel cinfill = ligr coutfill = yellow cconnect = vig; run;

The following statements request a u chart with a
fixed sample size of 30.0 for the control limits. In other words,
the control limits are computed as if each piece of fabric
were 30 meters long.
proc shewhart data=fabrics2; uchart defects*roll / subgroupn = sqmeters outlimits = flimits2 limitn = 30 alln nmarkers cinfill = ligr cconnect = cxfefefe cframe = steel; run;
The ALLN option specifies that points are to be displayed for all subgroups, regardless of their sample size. By default, when you specify the LIMITN= option, only points for subgroups whose sample size matches the LIMITN= value are displayed. The NMARKERS option requests special symbols that identify points for which the subgroup sample size differs from the nominal sample size of 30. The chart is shown in Output 41.3.4.
Output 41.3.4: Control Limits Based on Fixed Subgroup Sample SizeIn Output 41.3.4, no points lie outside the control limits, indicating that the process is in control. However, you should be careful when interpreting charts that use a nominal sample size, since the fixed control limits based on this value are only an approximation. Output 41.3.5 lists the data set FLIMITS2, which contains the fixed control limits displayed in Output 41.3.4.
Output 41.3.5: The Fixed Control Limits Data Set FLIMITS2

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