Quality and Reliability Methods - SAS

Chapter 5 Shewhart Control Charts 91
Phases
Levey-Jennings Charts
Levey-Jennings charts show a process mean with control limits based on a long-term sigma. The control
limits are placed at 3s distance from the center line.
The standard deviation, s, for the Levey-Jennings chart is calculated the same way standard deviation is in
the Distribution platform.
s
=
N
( y i
– y) 2
-------------------
N – 1
i = 1
Figure 5.16 Levey Jennings Chart
Phases
A phase is a group of consecutive observations in the data table. For example, phases might correspond to
time periods during which a new process is brought into production and then put through successive
changes. Phases generate, for each level of the specified Phase variable, a new sigma, set of limits, zones, and
resulting tests.
On the dialog for X -, R-, S-, IR-, P-, NP-, C-, U-, Presummarize, and Levey-Jennings charts, a Phase
variable button appears. If a phase variable is specified, the phase variable is examined, row by row, to
identify to which phase each row belongs.
Saving to a limits file reveals the sigma and specific limits calculated for each phase.