Example 20.4: Displaying Individual Measurements on an EWMA Chart

EWMACHART Statement

Example 20.4: Displaying Individual Measurements on an EWMA Chart

See MACEW5 in the SAS/QC Sample Library

In the manufacture of automotive tires, the diameter of the steel belts inside the tire is measured. The following data set contains these measurements for 30 tires:

   data tires;
      input sample diameter @@;
      datalines;
    1  24.05    2  23.99    3  23.95
    4  23.93    5  23.97    6  24.02
    7  24.06    8  24.10    9  23.98
   10  24.03   11  23.91   12  24.06
   13  24.06   14  23.96   15  23.98
   16  24.06   17  24.01   18  24.00
   19  23.93   20  23.92   21  24.09
   22  24.11   23  24.05   24  23.98
   25  23.98   26  24.06   27  24.02
   28  24.06   29  23.97   30  23.96
   ;

The following statements use the IRCHART statement in the SHEWHART procedure (see Chapter 34, "IRCHART Statement," in Part 9, "The SHEWHART Procedure") to create a data set containing the control limits for individual measurements and moving range charts for DIAMETER:

   proc shewhart data=tires;
      irchart diameter*sample / nochart
                                outlimits=tlimits;
   run;

A listing of the data set TLIMITS is shown in Output 20.4.1.

Output 20.4.1: Listing of the Data Set TLIMITS

Control Limits for Diameter Measurements

_VAR_	_SUBGRP_	_TYPE_	_LIMITN_	_ALPHA_	_SIGMAS_	_LCLI_	_MEAN_	_UCLI_	_LCLR_	_R_	_UCLR_	_STDDEV_
diameter	sample	ESTIMATE	2	.002699796	3	23.8571	24.0083	24.1596	0	0.056897	0.18585	0.050423

The upper and lower control limits for the diameter measurements are 24.1596 and 23.8571, respectively.

In this example, reference lines will be used to display the control limits for the individual measurements on the EWMA chart. The following DATA step reads these control limits from TLIMITS and creates a data set named VREFDATA, which contains the reference line information:

   data vrefdata;
      set tlimits;
      length _reflab_ $16.;
      keep _ref_ _reflab_;
      _ref_ = _lcli_; _reflab_= 'LCL for X'; output;
      _ref_ = _ucli_; _reflab_= 'UCL for X'; output;
   run;

A listing of the data set VREFDATA is shown in Output 20.4.2.

Output 20.4.2: Listing of the Data Set VREFDATA

Reference Line Information

_reflab_	_ref_
LCL for X	23.8571
UCL for X	24.1596

The following statements request an EWMA chart for these measurements:

   title 'EWMA Chart for Steel Belt Diameters';
   proc macontrol data=tires;
      ewmachart diameter*sample / weight      = 0.3
                                  meansymbol  = square
                                  lcllabel    = 'LCL for EWMA'
                                  ucllabel    = 'UCL for EWMA'
                                  vref        = vrefdata
                                  vreflabpos  = 3
                                  cframe      = vibg
                                  cinfill     = ligr
                                  cconnect    = salmon
                                  cmeansymbol = yellow;
   run;

The MEANSYMBOL= option displays the individual measurements on the EWMA chart. By default, these values are not displayed. The MEANSYMBOL= option specifies the symbol used to plot the individual measurements. The VREF= option reads the reference line information from VREFDATA. The resulting chart is shown in Output 20.4.3.

Output 20.4.3 indicates that the process is in control. None of the diameter measurements (indicated by squares) exceed their control limits, and none of the EWMAs exceed their limits.

Output 20.4.3: Displaying Individual Measurements on EWMA Chart

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