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 RCHART Statement

## Example 39.1: Computing Probability Limits

 See SHWREX1 in the SAS/QC Sample Library

This example demonstrates how to create R charts with probability limits. The following statements read the disk drive test times from the data set DISKS (see "Creating Range Charts from Raw Data" ) and create the R chart shown in Output 39.1.1:

   title 'Probability Limits for Disk Drive Test Times';
symbol v=dot c=rose;
proc shewhart data=disks;
rchart time*lot / alpha     = .01
outlimits = dlimits
cframe    = vipb
cinfill   = ywh
cconnect  = rose;
run;


The ALPHA= option specifies the probability () that a subgroup range exceeds its limits. Here, the limits are computed so that the probability that a range is less than the lower limit is ,and the probability that a range is greater than the upper limit is .This assumes that the measurements are normally distributed. The OUTLIMITS= option names an output data set that saves the probability limits. A listing of DLIMITS is shown in Output 39.1.2.

The variable _ALPHA_ saves the value of .Note that, in this case, the upper probability limit is equivalent to an upper limit.

Output 39.1.1: R Chart with Probability Limits

Output 39.1.2: Probability Limits Data Set

 Probablity Limits for Disk Drive Test Times

 _VAR_ _SUBGRP_ _TYPE_ _LIMITN_ _ALPHA_ _SIGMAS_ _LCLX_ _MEAN_ _UCLX_ _LCLR_ _R_ _UCLR_ _STDDEV_ time lot ESTIMATE 6 0.01 2.94715 7.95162 8.00307 8.05452 0.036645 0.124 0.24628 0.048927

Since all the points fall within the probability limits, it can be concluded that the variability in the disk drive performance is in statistical control.

The following statements apply the limits in DLIMITS to the times in the data set DISKS2 (see "Reading Preestablished Control Limits" ):

   title 'Probability Limits Applied to Second Set of Test Times';
symbol v=dot c=rose;
proc shewhart data=disks2 limits=dlimits;