BOXCHART Statement: SHEWHART Procedure

Saving Control Limits

Note: See Box Chart Examples in the SAS/QC Sample Library.

You can save the control limits for a box chart in a SAS data set; this enables you to apply the control limits to future data (see Reading Preestablished Control Limits) or modify the limits with a DATA step program.

The following statements read measurements from the data set Turbine (see Creating Box Charts from Raw Data) and save the control limits displayed in Figure 17.4 in a data set named TURBLIM:

proc shewhart data=Turbine;
   boxchart KWatts*Day / outlimits=Turblim
                         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 Turblim is listed in Figure 17.10.

Figure 17.10: The Data Set Turblim Containing Control Limit Information

Control Limits for Power Output Data

_VAR_ _SUBGRP_ _TYPE_ _LIMITN_ _ALPHA_ _SIGMAS_ _LCLX_ _MEAN_ _UCLX_ _LCLS_ _S_ _UCLS_ _STDDEV_
KWatts Day ESTIMATE 20 .002699796 3 3351.92 3485.41 3618.90 100.207 196.396 292.584 198.996


The data set Turblim contains one observation with the limits for process KWatts. The variables _LCLX_ and _UCLX_ contain the lower and upper control limits for the means, and the variable _MEAN_ contains the central line. The value of _MEAN_ is an estimate of the process mean, and the value of _STDDEV_ is an estimate of the process standard deviation $\sigma $. The value of _LIMITN_ is the nominal sample size associated with the control limits, and the value of _SIGMAS_ is the multiple of $\sigma $ 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 whether the values of _MEAN_ and _STDDEV_ are estimates or standard values.

The variables _LCLS_, _S_, and _UCLS_ are not used to create box charts, but they are included so that the data set Turblim can be used to create an s chart; see XSCHART Statement: SHEWHART Procedure. If you specify the RANGES option in the BOXCHART statement, the variables _LCLR_, _R_, and _UCLR_, rather than the variables _LCLS_, _S_, and _UCLS_, are included in the OUTLIMITS= data set. These variables can be used to create an R chart; see XRCHART Statement: SHEWHART Procedure.

If you specify CONTROLSTAT=MEDIAN to request control limits for medians, the variables _LCLM_ and _UCLM_, rather than the variables _LCLX_ and _UCLX_, are included in the OUTLIMITS= data set as demonstrated by the following statements:

proc shewhart data=Turbine;
   boxchart KWatts*Day / outlimits   = Turblim2
                         controlstat = median
                         nochart;
run;

Turblim2 is listed in Figure 17.11. For more information, see OUTLIMITS= Data Set.

Figure 17.11: The Data Set Turblim2 Containing Control Limit Information

Control Limits for Power Output Data

_VAR_ _SUBGRP_ _TYPE_ _LIMITN_ _ALPHA_ _SIGMAS_ _LCLM_ _MEAN_ _UCLM_ _LCLS_ _S_ _UCLS_ _STDDEV_
KWatts Day ESTIMATE 20 .002776264 3 3319.85 3481.63 3643.40 100.207 196.396 292.584 198.996


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

title 'Summary Statistics and Control Limit Information';
proc shewhart data=Turbine;
   boxchart KWatts*Day / outtable=Turbtab
                         nochart;
run;

The data set Turbtab is partially listed in Figure 17.12.

Figure 17.12: The OUTTABLE= Data Set Turbtab

Summary Statistics and Control Limit Information

_VAR_ Day _SIGMAS_ _LIMITN_ _SUBN_ _LCLX_ _SUBX_ _MEAN_ _UCLX_ _STDDEV_ _EXLIM_ _SUBMIN_ _SUBQ1_ _SUBMED_ _SUBQ3_ _SUBMAX_
KWatts 04JUL 3 20 20 3351.92 3487.40 3485.41 3618.90 198.996   3180 3340.0 3490.0 3610.0 4050
KWatts 05JUL 3 20 20 3351.92 3471.65 3485.41 3618.90 198.996   3179 3333.5 3419.5 3605.0 3849
KWatts 06JUL 3 20 20 3351.92 3488.30 3485.41 3618.90 198.996   3304 3376.0 3456.5 3604.5 3781
KWatts 07JUL 3 20 20 3351.92 3434.20 3485.41 3618.90 198.996   3045 3390.5 3447.0 3550.0 3629
KWatts 08JUL 3 20 20 3351.92 3475.80 3485.41 3618.90 198.996   2968 3321.0 3487.0 3611.5 3916
KWatts 09JUL 3 20 20 3351.92 3518.10 3485.41 3618.90 198.996   3047 3425.5 3576.0 3615.0 3881
KWatts 10JUL 3 20 20 3351.92 3492.65 3485.41 3618.90 198.996   3002 3368.5 3495.5 3621.5 3787
KWatts 11JUL 3 20 20 3351.92 3496.40 3485.41 3618.90 198.996   3196 3346.0 3473.5 3592.5 3994
KWatts 12JUL 3 20 20 3351.92 3398.50 3485.41 3618.90 198.996   3115 3188.5 3426.0 3568.5 3731
KWatts 13JUL 3 20 20 3351.92 3456.05 3485.41 3618.90 198.996   3263 3340.0 3444.0 3505.5 4040
KWatts 14JUL 3 20 20 3351.92 3493.60 3485.41 3618.90 198.996   3215 3336.0 3441.5 3616.0 3872
KWatts 15JUL 3 20 20 3351.92 3563.30 3485.41 3618.90 198.996   3182 3409.5 3561.0 3719.5 3850
KWatts 16JUL 3 20 20 3351.92 3519.05 3485.41 3618.90 198.996   3212 3378.0 3515.0 3682.5 3769
KWatts 17JUL 3 20 20 3351.92 3474.20 3485.41 3618.90 198.996   3077 3329.0 3501.5 3599.5 3812
KWatts 18JUL 3 20 20 3351.92 3443.60 3485.41 3618.90 198.996   3061 3315.5 3435.0 3614.5 3815
KWatts 19JUL 3 20 20 3351.92 3586.35 3485.41 3618.90 198.996   3288 3426.5 3546.0 3762.5 3877
KWatts 20JUL 3 20 20 3351.92 3486.45 3485.41 3618.90 198.996   3114 3373.0 3474.5 3635.5 3928
KWatts 21JUL 3 20 20 3351.92 3492.90 3485.41 3618.90 198.996   3167 3400.5 3488.0 3582.5 3801
KWatts 22JUL 3 20 20 3351.92 3432.80 3485.41 3618.90 198.996   3056 3322.0 3460.0 3561.0 3800
KWatts 23JUL 3 20 20 3351.92 3496.90 3485.41 3618.90 198.996   3145 3308.5 3495.0 3652.0 3917


This data set contains one observation for each subgroup sample. The variable _SUBMIN_ contains the subgroup minimums, and the variable _SUBQ1_ contains the first quartile for each subgroup. The variable _SUBX_ contains the subgroup means, and the variable _SUBMED_ contains the subgroup medians. The variable _SUBQ3_ contains the third quartiles, and the variable _SUBMAX_ contains the subgroup maximums. The variable _SUBN_ contains the subgroup sample sizes. The variables _LCLX_ and _UCLX_ contain the lower and upper control limits for the means. 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.

An OUTTABLE= data set can be read later as a TABLE= data set. For example, the following statements read Turbtab and display a box chart (not shown here) identical to the chart in Figure 17.4:

title 'Box Chart for Power Output';
proc shewhart table=Turbtab;
   boxchart KWatts*Day;
label _SUBX_ = 'Average Power Output';
run;

Because the SHEWHART procedure simply displays the information in a TABLE= data set, you can use TABLE= data sets to create specialized control charts (see Specialized Control Charts: SHEWHART Procedure).

For more information, see TABLE= Data Set.