XCHART Statement: CUSUM Procedure

Reading Cusum Scheme Parameters

See CUSONES1 in the SAS/QC Sample LibraryThis example shows how the cusum parameters saved in the previous example can be applied to new measurements saved in a data set named Cans2:

data Cans2;
   length pump $ 8;
   label Hour = 'Hour';
   input Hour Weight pump $ 8. ;
   datalines;
16  8.1765  Pump 3
17  8.0949  Pump 3
18  8.1393  Pump 3
19  8.1491  Pump 3
20  8.0473  Pump 1
21  8.1602  Pump 1
22  8.0633  Pump 1
23  8.0921  Pump 1
24  8.1573  Pump 1
25  8.1304  Pump 1
26  8.0979  Pump 1
27  8.2407  Pump 1
28  8.0730  Pump 1
29  8.0986  Pump 2
30  8.0785  Pump 2
31  8.2308  Pump 2
32  8.0986  Pump 2
33  8.0782  Pump 2
34  8.1435  Pump 2
35  8.0666  Pump 2
;

The following statements create a one-sided cusum chart for the measurements in Cans2 using the parameters in cusparm:

title "One-Sided Cusum Analysis for New Data";
proc cusum data=Cans2 limits=cusparm;
   xchart Weight*Hour( pump ) / odstitle=title markers;
   label Weight = 'Cusum of Weight';
run;
ods graphics off;

ODS Graphics remains enabled until it is disabled with the ODS GRAPHICS OFF statement, so this cusum chart is also created using ODS Graphics.

The LIMITS= option in the PROC CUSUM statement specifies the data set containing preestablished cusum parameters.^[28] The chart, shown in Figure 6.12, indicates that the process is in control. Levels of the variable pump (referred to as a block-variable) do not enter into the analysis but are displayed in a block legend across the top of the chart. See Block Variable Legend Options.

Figure 6.12: Cusum Chart with Decision Interval for New Data

In general, the parameters for a specified process and subgroup-variable are read from the first observation in the LIMITS= data set for which

the value of _VAR_ matches the process (in this case, Weight)
the value of _SUBGRP_ matches the subgroup-variable name (in this case, Hour)

If you are maintaining more than one set of cusum parameters for a particular process, you will find it convenient to include a special identifier variable named _INDEX_ in the LIMITS= data set. This must be a character variable of length 16. Then, if you specify READINDEX=’value’ in the XCHART statement, the parameters for a specified process and subgroup-variable are read from the first observation in the LIMITS= data set for which

the value of _VAR_ matches process
the value of _SUBGRP_ matches the subgroup-variable name
the value of _INDEX_ matches value

In this example, the LIMITS= data set was created in a previous run of the CUSUM procedure. You can also create a LIMITS= data set with the DATA step. See LIMITS= Data Set for details concerning the variables that you must provide.

^[28]In SAS 6.09 and in earlier releases, it is also necessary to specify the READLIMITS option or the READINDEX= option to read cusum parameters from a LIMITS= data set.