If the data for a Shewhart chart can be classified by factors relevant to the process (for instance, machines or operators), displaying the classification on the chart can facilitate the identification of special or common causes of variation that are related to the factors. Kume (1985) refers to this type of classification as "stratification" and describes various ways to create stratified control charts.
There are important differences between stratification and subgrouping. The data must always be classified into subgroups before a control chart can be produced. Subgrouping affects how control limits are computed from the data as well as the outcome of tests for special causes (see Tests for Special Causes: SHEWHART Procedure). The values of the subgroup-variable specified in the chart statement classify the data into subgroups. In contrast, stratification is optional and involves classification variables other than the subgroup-variable. Displaying stratification influences how the chart is interpreted, but it does not affect control limits or tests for special causes.
This section describes three types of variables that you can specify to create stratified control charts.
A symbol-variable stratifies data into levels of a classification variable.
The block-variables stratify data into blocks of consecutive observations.
A _PHASE_ variable stratifies data into time phases.
You can specify any combination of these three variables. You should be careful, however, because it is possible to generate confusing charts by overusing these methods.
The data for the examples in this section consist of diameter measurements for a part produced on one of three different machines.
Three subgroups, each consisting of six parts, are sampled each day, corresponding to three shifts worked each day. The data
are provided in the data set Parts, which is created by the following statements:
data Parts; length Machine $ 4; input Sample Machine $ Day Shift DiamX DiamS; DiamN=6; datalines; 1 A386 01 1 4.32 0.39 2 A386 01 2 4.49 0.35 3 A386 01 3 4.44 0.44 4 A386 02 1 4.45 0.17 5 A386 02 2 4.21 0.53 6 A386 02 3 4.56 0.26 7 A386 03 1 4.63 0.39 8 A386 03 2 4.38 0.47 9 A386 03 3 4.47 0.40 10 A455 04 1 4.42 0.37 11 A455 04 2 4.45 0.32 12 A455 04 3 4.62 0.36 13 A455 05 1 4.33 0.31 14 A455 05 2 4.29 0.33 15 A455 05 3 4.17 0.25 16 C334 08 1 4.15 0.28 17 C334 08 2 4.21 0.33 18 C334 08 3 4.16 0.19 19 C334 09 1 4.14 0.13 20 C334 09 2 4.11 0.19 21 C334 09 3 4.10 0.27 22 C334 10 1 3.99 0.14 23 C334 10 2 4.24 0.16 24 C334 10 3 4.23 0.14 25 A386 11 1 4.27 0.28 26 A386 11 2 4.70 0.45 27 A386 11 3 4.51 0.45 28 A386 12 1 4.34 0.16 29 A386 12 2 4.38 0.29 30 A386 12 3 4.28 0.24 31 A386 15 1 4.47 0.26 32 A386 15 2 4.31 0.46 33 A386 15 3 4.52 0.33 ;