Page 256 - Lean six sigma demystified

234 Lean Six Sigma DemystifieD

Piston heads, for example, will have specifications for the maximum and
minimum height and diameter of the head; roundness of the head, known as
cylindricity and concentricity; and a host of other factors like how the shaft con-
nects to the piston and so on. A bottle will have similar specifications. Usually, a
manufactured part will have both an upper and lower specification limit.
For most services, customers may have an upper limit, but no lower limit.
Teller wait times and call center wait times will usually only have a maximum
time (the minimum time is automatically zero). Most customers don’t want
to wait longer than 5 minutes in a bank and no more than 30 seconds on the
phone. These customers have an upper specification limit, but no lower limit
other than zero. Go into any fast-food restaurant and you’ll see a little digital
timer ticking away next to your order. Fast-food restaurants can’t afford to be
slow, because customers are paying for speed and convenience.
There are rare instances where you will have only a lower specification limit
but no upper specification limit.

Causes of Variation

Within these specification limits, there are two causes of variation.
1. Special causes (i.e., assignable causes of special events). Special cause varia-
tion can be easily detected with control charts, quickly analyzed with the
five whys, and corrected by the operator. Special cause variation accounts
for only 15% of the problems. Most companies get caught up in firefighting
the special causes, but rarely get around to reducing the common causes.
2. Common causes are factors affecting the whole system. These will require

some deeper root cause analysis. Common causes account for 85% of the
total variation. Think about your drive to work. Common causes of varia-
tion in your commute time might include time of day, number of red
traffic lights, number of cars on the road, and road construction or main-
tenance. Most weather conditions fall under common cause variation, but
in Denver we occasionally get a blizzard and it can take four times as long
to get to work. A Denver blizzard is a special cause of variation. Rain in
Seattle would be a common cause, because it rains there often. Although
I can’t change traffic lights or prevent a blizzard, I did find that if I left
15 minutes earlier on normal days and left an hour earlier on snow days,
I could shorten my commute by 10 minutes on normal days and 90 minutes
on snow days. Why? Because there were fewer cars on the road. Since I