Page 180 - Six Sigma Demystified

Chapter 7 i m p r o v e S tag e 161

The severity, likelihood of occurrence, detection method, and likelihood of
detection are determined, and a resulting risk factor (risk priority number, or
RPN) is calculated. The likelihood of occurrence and likelihood of detection
are based on historical data, where available.
The results of the FMEA will indicate activities prone to failure or likely to
cause serious consequences if they do fail. These failure modes must be
addressed, either through elimination or via a mitigation strategy. Improve-
ments in detection are stopgap measures with increased cost of quality. Reduc-
ing the risk often demands a reduction in the rate of occurrence, such as through
the process optimization techniques described earlier. Prevention methods can
achieve satisfactory results, particularly in reducing the rate of occurrence of
human errors.
Figure 7.4 displays a PDPC analysis of the key process steps, as indicated by the
RPN calculated in the FMEA for each process step. The contingency plans noted
for each step result in a corresponding decrease in RPN noted in the figure.

Prevention of Human errors

It is useful to categorize process failures according to their origin. Many failures
are due to human error, particularly in service processes. While some solutions
will focus on the performance of individuals, most failures resulting from
human error will be prevented by process or system- level solutions.
There are three main categories of human errors: inadvertent errors, tech-
nique errors, and willful errors. Inadvertent errors otherwise may be termed
mistakes. Technique errors are related to the process procedure and often are due
to poor training. Willful errors are deliberate attempts to sabotage the process.

Inadvertent errors typically are characterized by a low incidence rate, with
little or no advance knowledge that a failure is coming. There is no predictable
pattern to an inadvertent error.
Inadvertent errors can be prevented in a number of ways. Foolproofing, also
called poka yoke, is one of the lean tools for preventing errors. A fundamental
change is incorporated into the design of the part or the process to prevent the
error from occurring.
For example, modern hypodermic needles now have been designed to pre-
vent the needle from being used more than once to avoid possible instances of
cross- contamination between patients. To prevent holes from being drilled in
the wrong place on a production part, the part and the fixture used to secure
the part could incorporate a pin with a mating slot so that the part will not fit
correctly into the fixture unless it is aligned properly.