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278 Part IV: Quality Assurance
• The process performance has improved or it is improving. This
condition of the process should be investigated very carefully
so that such conditions of improvement are implemented on a
permanent basis at this location and elsewhere in the industry.
• The measurement system has changed.
5. As with the X – and R and X – and S charts, the presence of any unusual
patterns or trends is either an indication of an unstable process or it
is an advance warning of conditions that, if left unattended or without
any appropriate action, could make the process unstable.
6. If p – is moderately high (np 5) then an approximately equal number of
points should fall on either side of the center line. Therefore, either of
the following conditions could indicate that the process has shifted
or a trend of shift has started:
• A run of seven or more points going up or going down
• A run of seven or more points falling either below or above the
center line
7. A run above the center line or a run going up generally indicates:
• The process performance has deteriorated and may still be
deteriorating
• The measurement system has changed
8. A run below the center line or a run going down generally indicates:
• The process performance has improved and may still be
improving
Part IV.B.4
• The measurement system has changed
To illustrate the construction of the p chart we will consider the data from Example
19.6, shown in Table 19.5.
EXAMPLE 19.6
A semiconductor manufacturer tracks the number of nonconforming computer chips
produced each day. A team of Six Sigma Green Belts wants to improve the overall quality
by reducing the fraction of nonconforming computer chips. To achieve this goal, the
team decided to set up a p chart based on daily inspections of 1000 chips over a period
of 30 days. Table 19.5 gives the number of nonconforming chips out of 1000 inspected
chips each day during the study period of 30 days.
Solution:
Using the data in Table 19.5 we develop the trial control limits of the p chart as follows:
Continued