192 THETOYOTAWAYFIELDBOOK
proofing—to post notification of the rule or method—is also rarely
effective. Most people assume that a sign, clearly in view, will prevent
errors. This seems logical. People don’t break the rules maliciously
(most of the time), but they often rationalize, “I don’t think I’ll
make the mistake, so it’s okay if I break the rule.”
3. Label the paint lines. Overspray buildup quickly obliterated any
labels and made them unreadable.
4. A cover “flap” was placed over the QD for the white paint line,
which required the painter to lift the cover to reconnect the hose.
This countermeasure was based on the fact that the majority of
incidences in the past involved white paint mixing with another
color. Since 40 percent of all vehicles were white, the odds of having
multiple white bumpers within the sequence simultaneously were
higher. The cover over the white QD was intended to make the
operator “think” before replacing the white paint line (similar to a
yoshi). This “solution” also had no effect because it did not prevent
connecting the wrong lines. It only made the work more difficult
(lifting the cover for 40 percent of the jobs).
These four attempts at prevention represent the hierarchy of error
proofing moving from telling or sharing information, to posting
notices, to attempts at prevention by self-checking. The efforts may
have prevented some occurrences, but they did not prevent them all.
After these attempts to eliminate the error failed, a solution was pro-
posed to use a device known as a “peanut.” This allowed the paint to
recirculate at the paint gun, and eliminated the need to disconnect
the paint line. This was an effective preventive device, but the negative
points were additional weight on the paint gun (nearly one pound),
which was an ergonomic concern, and the high cost of the devices.
An installation on the entire paint line would cost over $10,000—not
a low-cost solution.
It was clear from observation that the operators’ inclination to deviate
from the described method was not out of spite. They were deviating
because of a natural desire to reduce their own burden, and they
assumed that they would never make the mistake. In this case, an error-
proofing method was needed that removed any need for a conscious
act (following the rules). It was necessary to remove any options.
Watching the painters disconnect and reconnect the line, it looked
like someone putting a key in a lock and opening a door. This was the
seed for an idea. What if each paint line could somehow be made to
fit only one QD? What if each QD and paint-line combination could
be like a lock and a key? The QDs would need to be like locks, and
the paint lines keys. A toolmaker was able to make a mock-up sample
