Operations and Supply Chain Management The Core

322 OPERATIONS AND SUPPLY CHAIN MANAGEMENT
exhibit 10.11
Process Control Chart Evidence for Investigation
Upper control limit
Central line
Lower control limit
Upper control limit
Normal behavior.
One point out above.
Investigate for cause of high
value.
One point out below.
Investigate for cause of
low value.
Central line
Lower control limit
Upper control limit
Two points near upper
control. Investigate for cause
of poor performance.
Two points near lower
control. Investigate for
cause.
Run of five above central line.
Investigate for cause of
sustained poor performance.
Central line
Lower control limit
Upper control limit
Run of five below central line.
Investigate for cause of
change.
Trend in either direction.
Investigate for cause of
progressive change.
Erratic behavior.
Investigate.
Central line
Lower control limit
Sudden change in level.
Investigate for cause.
Time Time Time
where ​ ¯ p ​ is the fraction defective, s p is the standard deviation, n is the sample size, and z is the
number of standard deviations, the Z-score, for a specific confidence. Typically, z = 3
(99.7 percent confidence) or z = 2.58 (99 percent confidence) is used.
Size of the Sample The size of the sample must be large enough to allow counting of the
attribute. For example, if we know that a machine produces 1 percent defective units, then
a sample size of five would seldom capture a bad unit. A rule of thumb when setting up a
p-chart is to make the sample large enough to expect to count the attribute twice in each