Page 194 - Lean six sigma demystified

Chapter 5 Redu C ing d efe C t S with Six Sigm a 173

a map or diagram with hash marks on it or a matrix of stroke tallies. Have your
doers make a mark every time they encounter a certain type of problem.
In Edward Tufte’s book, Visual Explanations, he recounts the analysis of Lon-
don’s cholera epidemics in the 1850s as originally described in John Snow’s book,
On the Mode of Communication of Cholera. Snow suspected that water pumps in
London were the cause of cholera outbreaks (you had to pump water and carry it
to your house).
When cholera broke out in September 1854, Snow took a street diagram of
London and started marking where each death occurred. The deaths clustered
around the pump on Broad Street. Snow analyzed the water, but couldn’t see
any obvious impurities. Snow realized that “the absence of evidence was not
evidence of absence.”
Snow took his death diagram to the city board, and they immediately
removed the pump handle on the Broad Street pump. Cholera deaths began to
decline immediately. Snow chased down every oddball cholera death and found
that people from as far away as Chelsea got their water and their cholera from
the Broad Street pump.
If you produce a circuit board, can you imagine having a diagram of the
board and putting a mark on each component when you find a failure? In a
manufacturing plant, can you imagine a floor plan of the production facility
and putting a mark on the chart every time a machine breaks down or a prob-
lem occurs? If you’re an information system developer, can you imagine mak-
ing a stroke tally for every trouble report or enhancement request to identify
the 4% of your code that requires most of the repair or enhancement work?
Check sheets can be your friend when you don’t have enough data.

Root Cause Analysis

For every thousand hacking at the leaves of evil, there is one striking at the root.
—David Thoreau

The Ishikawa, cause-and-effect, or fishbone diagram helps work backward to
diagnose root causes. For those unfamiliar with root cause analysis, learning
to use the fishbone can be frustrating, but, once learned, it helps prevent
knee-jerk, symptom patching. There are two main types of fishbone dia-
grams. One is a customized version of the generic—people, process, machines,
materials, measurement, and environment (Fig. 5-27). The other is a step-by-
step, process fishbone that begins with the first step and works backward
(Fig. 5-28), because errors early in the process often cause the biggest effects.