Page 196 - Six Sigma Demystified

176 Six SigMa DemystifieD

and even multiple outputs. The inputs are often sampled at high frequencies,
and once an acceptable model is discovered, the process can be controlled to a
high level of precision.
Since EPC makes changes to the process based on the current state of the
process, the data are inherently autocorrelated, or dependent on earlier observa-
tions. This serial correlation violates the assumption of data independence
required for standard SPC control charts and must be addressed if control
charts are to be used to monitor process output. This is discussed in more detail
in the “Auto Correlation” charts section of Part 3.

Operational Procedures

As with EPC and, to some extent, SPC, operational procedures seek to control
the output of the process using operational guidelines for the human inputs to
the process. Rather than automatically controlling temperature using a feed-
back control such as a thermostat, operational procedures would instruct a
process operator to turn on the heater or air conditioner when the temperature
reached a particular setting. It should be clear that detection of the condition,
as well as the response to the condition, is not as repeatable as when using au-
tomated control. The procedures are subject to human error, as discussed in
Chapter 7.
Nonetheless, procedures offer the most cost-effective means of control for
many circumstances. Particularly in transactional processes, procedures may be
used effectively to
• Standardize the process. For example, using flowcharts or decision trees,

each of the order-processing clerks will process the orders in the same
fashion. Simple software also can be created to facilitate the process.
• Divert resources. As a matter of procedure, personnel are instructed that if
the wait for their service is more than five minutes, then additional re-
sources should be requested immediately. For example, if there are more
than three people in your checkout line, request additional cashiers to the
front of the store.
• Channel orders by types. Certain customers or types of orders are routed
to a standardized process line for improved efficiency. For example, at an
automotive service department, the oil-change customers are routed to a
special line for expedited service. At a bank, an express lane is provided
for merchant accounts.