Quality Progress - January 2016 - (Page 27)
That's So Random-or Is It?
ssignable cause, random variation, process stability, process
capability-these terms can easily confuse those new to process management. Does random variation in a
process automatically require process
improvement? Can a process be stable
and capable with an assignable cause
present? Not knowing the answers to
questions like these can cause process
owners to overengineer and possibly
overcorrect their processes.
A simple example of random variation is the process of driving to work.
Let's say your typical drive time is 20
minutes, but it's not exactly 20 minutes every day. There are random
variables or causes that impact your
drive time, including the speed at
which you drive, traffic patterns and
the time you leave the house. So the
random process variation may extend
from 18 to 25 minutes.
One day, however, your drive time
increases to 55 minutes (see Figure 1).
You investigate the cause and determine there was an accident, which
caused the process to spike out of control. The accident is the assignable
cause of the out-of-control situation.
In other words, you know what happened and can assign a cause to the
out-of-control situation in the process.
Even though you know what caused
the increase in drive time, you
wouldn't revise the process because
of this one assignable cause.
The same theory applies to business
process management. You need to
Identify and investigate
establish a random process variance
for each of the processes for which
you're responsible, such as the drive
time variance of 18 to 25 minutes. You
need to know what you can expect
and plan for every day. Then, when
random, assignable cause incidents
that negatively impact the process
occur, you need to investigate the
cause, assign a cause, document the
cause and continue to monitor process
An assignable cause log (see Figure
2) can effectively document assignable cause incidents, allowing you to
capture the information for use in
process performance reviews or
If an assignable
or recurring, you
apply a process
Mon. Tues. Wed. Thurs. Fri. Mon. Tues. Wed. Thurs. Fri.
bring the process
back into control
or to the previous
of process variaFIGURE 2
Assignable Cause Log
tion. To illustrate
this point, let's
Control plan measure: drive time
say your drive
time increased to
55 minutes per
cause detail Action taken
day for five days
5/23/05 Drive time
Investigation revealed an isolated
in a row, causing
increased to incident-an accident occurred.
to be late for
No improvement needed at this time.
work all week,
I MARCH 2006 I www.asq.org
by Cecelia McCain
which is unacceptable. Now you have
a chronic, recurring, out-of-control situation, so you need to investigate the
cause and implement a process
After investigating the cause, you
determine the train schedule changed,
which negatively impacted the flow
of traffic. Therefore, you need to
adjust or improve the process to
remove the now chronic assignable
cause. Simple improvements include
changing your driving route or work
schedule or leaving home earlier or
later to mitigate the negative impact
of the changed train schedule on your
driving time. More complex and drastic improvements include selling your
home or changing jobs.
Random cause variation is inherent
in every process, from simple order
taking to complex automobile assembly. Look at the processes in your
own organization and identify the
extent to which random variation
exists by plotting the data over time.
Use simple control charts and establish upper and lower control limits to
maintain a stable process.
Assignable cause will occur in every
process sooner or later. The key to
effective process management and
stability is to investigate and identify
all assignable cause incidents, while
remembering you don't need to make
process improvements after every
CECELIA MCCAIN is the director of quality
program management at Compuware Corp. in
Detroit. She is a member of ASQ and a certified Six Sigma Black Belt.
If you would like to comment on
this article, please post your remarks
on the Quality Progress Discussion
Board at www.asq.org, or e-mail
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January 2016 * QP 27
Table of Contents for the Digital Edition of Quality Progress - January 2016
According to Plan
Use Your Head
Stakeholder Management 101
All About Data
Eight Simple Steps
Which Six Sigma Metric Should I Use?
Turning ‘Who’ Into ‘How’
In the Beginning
Outputs and Outcomes
That’s So Random—Or Is It?
Improving a System
Putting It All on the Table
Know the Drill
It’s Fun To Work With an F-M-E-A
Solve Problems With Open Communication
Tell Me About It
Separate the Vital Few From the Trivial Many
To DMAIC or Not to DMAIC?
Breaking It Down
1 + 1 = Zero Defects
Curve Your Enthusiasm
Make a Choice
What Is a Fault Tree Analysis?
Successful Relationship Diagrams
The Benefits of PDCA
Return on Investment
The Art of Root Cause Analysis
Why Ask Why?
Get to the Root of It
Checks and Balances
Clearing SPC Hurdles
Supplier Selection and Maintenance
Building a Quality Team
Plan Experiments to Prevent Problems
Quality Progress - January 2016