Applying Tests with Multiple Phases

Tests for Special Causes

Applying Tests with Multiple Phases

See SHWTSC1 in the SAS/QC Sample Library

The data set ASSEMBLY includes a variable named SYSTEM, which indicates the manufacturing system used to produce each assembly. As shown by the following statements, this variable can be temporarily renamed and read as the variable _PHASE_ to create a control chart that displays the phases (groups of consecutive subgroups) for which SYSTEM is equal to T, R, and Q:

   symbol v=dot c=salmon;
   title 'Manufacturing Systems Used in Assembly';
   proc shewhart
      history=assembly (rename=(system=_phase_));
      xrchart offset * sample /
         mu0         = 20
         sigma0      = 2.24
         limitn      = 5
         alln
         tests       = 1 to 4
         testlabel   = ( comment )
         ltests      = 2
         readphases  = ('T' 'R' 'Q')
         phaselegend
         phaseref
         vaxis       = 16 to 26 by 2
         split       = '/'
         cinfill     = ywh
         cframe      = vligb
         cconnect    = salmon
         ctests      = black;
      label offsetx = 'Avg Offset in cm/Range';
   run;

The chart is shown in Figure 48.7.

Figure 48.7: Single Set of Limits with Multiple Phases

Note that a single set of fixed 3 $\sigma$ limits is displayed for all three phases because LIMITN=5 and ALLN are specified. Consequently, the tests requested with the TESTS= option are applied independently of the phases. In general, however, it is possible to display distinct sets of control limits for different phases, and in such situations, the tests are not applied independently of phases, as discussed in the next example.

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