Page 197 - Six Sigma Demystified

Chapter 8 co n t r o l S tag e 177

Operational procedures must be documented and controlled to ensure
their use. Control of procedures refers to revision control, where operational
personnel have access only to current (not outdated) procedures that have
been approved by the appropriate stakeholders. The ISO 9000 provisions for
document control are an effective means of ensuring proper revision
control.
Effective procedures may be documented in paragraph form, with flow-
charts or process maps, with pictures, or with any combination of these. The
documentation method and terminology should be appropriate for the audi-
ence. Bear in mind the objective: to effectively communicate the proper pro-
cedure. For personnel who have a limited command of the local language,
procedures in paragraph form may be completely ineffective. Simple pictures
showing the correct and incorrect methods may be the most effective way to
communicate the procedure to that audience, whereas paragraph form may
be used for another audience for the same procedure.
Each of these process control techniques (SPC, EPC, and operational proce-
dures) requires a necessary standard of performance, feedback to the process,
and a method of changing the process. These parameters should be specified in
a control plan.

Control Plans

A control plan provides an overview of the strategies that will be used to ensure
that key process or part characteristics are controlled through either detection
or prevention strategies or a combination of the two.
Key inputs to a control plan include the results of designed experiments

and FMEA. Designed experiments are fundamental tools in determining pro-
cess drivers. Robust designs in particular tell us which process factors must be
controlled to reduce variation in the process response. By controlling these
input factors, we can prevent errors from occurring in the process. As shown
in Chapters 6 and 7, these methods can be applied effectively to transactional
processes.
FMEA [and its risk priority number (RPN)] determines the failure modes
that are the most critical to control. This is a valuable input in construction of
the control plan. The assumptions used to determine the detection level in the
FMEA must be incorporated into the control plan.
A portion of a control plan is shown in Table 8.1. For each key characteristic,
the following information is defined: