Example 36.1: Computing Probability Limits

RCHART Statement

Example 36.1: 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 36.1.1:

   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 ( $\alpha$ ) 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 $\alpha/2=0.005$ ,and the probability that a range is greater than the upper limit is $\alpha/2=0.005$ .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 36.1.2.

The variable _ALPHA_ saves the value of $\alpha$ .Note that, in this case, the upper probability limit is equivalent to an upper $2.95\sigma$ limit.

Output 36.1.1: R Chart with Probability Limits

Output 36.1.2: 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;
      rchart time*lot / readalpha
                        cframe   = vipb
                        cinfill  = ywh
                        cconnect = rose;
   run;

The READALPHA option^* specifies that the variable _ALPHA_, rather than the variable _SIGMAS_, is to be read from the LIMITS= data set. Thus the limits displayed in the chart, shown in Output 36.1.3, are probability limits.

Output 36.1.3: Reading Probability Limits from a LIMITS= Data Set

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