Saving Control Limit Parameters

[See MACMA1 in the SAS/QC Sample Library]You can save the control limit parameters used for a moving average chart in a SAS data set; this enables you to use these parameters with future data (see Reading Preestablished Control Limit Parameters) or modify the parameters with a DATA step program.

The following statements read measurements from the data set Clips1 (see Creating Moving Average Charts from Raw Data) and save the control limit parameters in a data set named Cliplim:

title 'Control Limit Parameters';
proc macontrol data=Clips1;
   machart Gap*Day / span      = 3
                     outlimits = Cliplim
                     nochart;
run;

The OUTLIMITS= option names the data set containing the control limits, and the NOCHART option suppresses the display of the chart. The data set Cliplim is listed in Figure 9.15.

Figure 9.15 The Data Set Cliplim Containing Control Limit Information
Control Limit Parameters

_VAR_ _SUBGRP_ _TYPE_ _LIMITN_ _ALPHA_ _SIGMAS_ _MEAN_ _STDDEV_ _SPAN_
Gap Day ESTIMATE 5 .002699796 3 14.95 0.21108 3

Note that the data set Cliplim does not contain the actual control limits, but rather the parameters required to compute the limits.

The data set contains one observation with the parameters for process Gap. The variable _SPAN_ contains the number of terms used to calculate the moving average. The value of _MEAN_ is an estimate of the process mean, and the value of _STDDEV_ is an estimate of the process standard deviation . The value of _LIMITN_ is the nominal sample size associated with the control limits, and the value of _SIGMAS_ is the multiple of associated with the control limits. The variables _VAR_ and _SUBGRP_ are bookkeeping variables that save the process and subgroup-variable. The variable _TYPE_ is a bookkeeping variable that indicates that the values of _MEAN_ and _STDDEV_ are estimates rather than standard values. For more information, see OUTLIMITS= Data Set.

You can create an output data set containing the control limits and summary statistics with the OUTTABLE= option, as illustrated by the following statements:

title 'Summary Statistics and Control Limits';
proc macontrol data=Clips1;
   machart Gap*Day / span     = 3
                     outtable = Cliptab
                     nochart;
run;

The data set Cliptab is listed in Figure 9.16.

This data set contains one observation for each subgroup sample. The variable _UWMA_ contains the uniformly weighted moving average. The variables _SUBX_, _SUBS_, and _SUBN_ contain the subgroup means, subgroup standard deviations, and subgroup sample sizes, respectively. The variables _LCLA_ and _UCLA_ contain the lower and upper control limits, and the variable _MEAN_ contains the central line. The variables _VAR_ and Day contain the process name and values of the subgroup-variable, respectively. For more information, see OUTTABLE= Data Set.

Figure 9.16 The OUTTABLE= Data Set Cliptab
Summary Statistics and Control Limits

_VAR_ Day _SIGMAS_ _LIMITN_ _SPAN_ _SUBN_ _SUBX_ _SUBS_ _LCLA_ _UWMA_ _MEAN_ _UCLA_ _STDDEV_ _EXLIM_
Gap 1 3 5 3 5 14.904 0.18716 14.6668 14.9040 14.95 15.2332 0.21108  
Gap 2 3 5 3 5 15.014 0.09317 14.7498 14.9590 14.95 15.1502 0.21108  
Gap 3 3 5 3 5 14.866 0.25006 14.7865 14.9280 14.95 15.1135 0.21108  
Gap 4 3 5 3 5 15.048 0.23732 14.7865 14.9760 14.95 15.1135 0.21108  
Gap 5 3 5 3 5 15.024 0.26792 14.7865 14.9793 14.95 15.1135 0.21108  
Gap 6 3 5 3 5 15.126 0.12260 14.7865 15.0660 14.95 15.1135 0.21108  
Gap 7 3 5 3 5 15.220 0.23098 14.7865 15.1233 14.95 15.1135 0.21108 UPPER
Gap 8 3 5 3 5 14.902 0.17254 14.7865 15.0827 14.95 15.1135 0.21108  
Gap 9 3 5 3 5 14.910 0.19824 14.7865 15.0107 14.95 15.1135 0.21108  
Gap 10 3 5 3 5 14.932 0.24035 14.7865 14.9147 14.95 15.1135 0.21108  
Gap 11 3 5 3 5 15.096 0.25618 14.7865 14.9793 14.95 15.1135 0.21108  
Gap 12 3 5 3 5 14.912 0.16903 14.7865 14.9800 14.95 15.1135 0.21108  
Gap 13 3 5 3 5 15.138 0.15928 14.7865 15.0487 14.95 15.1135 0.21108  
Gap 14 3 5 3 5 14.798 0.26329 14.7865 14.9493 14.95 15.1135 0.21108  
Gap 15 3 5 3 5 14.944 0.20876 14.7865 14.9600 14.95 15.1135 0.21108  
Gap 16 3 5 3 5 14.896 0.09965 14.7865 14.8793 14.95 15.1135 0.21108  
Gap 17 3 5 3 5 14.734 0.22512 14.7865 14.8580 14.95 15.1135 0.21108  
Gap 18 3 5 3 5 15.046 0.24141 14.7865 14.8920 14.95 15.1135 0.21108  
Gap 19 3 5 3 5 14.702 0.17880 14.7865 14.8273 14.95 15.1135 0.21108  
Gap 20 3 5 3 5 14.788 0.16634 14.7865 14.8453 14.95 15.1135 0.21108  

An OUTTABLE= data set can be read later as a TABLE= data set. For example, the following statements read Cliptab and display a moving average chart (not shown here) identical to Figure 9.11:

title 'Moving Average Chart for Gap Measurements';
proc macontrol table=Cliptab;
   machart Gap*Day;
run;

For more information, see TABLE= Data Set.