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In-Spec vs. In-Control |
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by
Ray Mikulak |
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 Is
there a difference between being in-control and in-spec? Yes
there is, and it is a big difference. You can be
in-spec but not in-control. And you can be in-control but
not in-spec.
So, if you can
have only one, which is better - being in-spec or
in-control? Some may say “in-spec of course.”
But unless the spec is very generous and forgiving, you will
always be better off with a process that is in-control, but
not completely in-spec. Obviously neither is the ideal situation - but there is a
fundamental problem with a process that is in-spec, but not
in-control. The problem is that you never know from
one minute to the next if it will go out-of-spec. This
means that you must inspect every part or product that is
made - otherwise, how will you know that the product is
still in-spec?
So, what is the
advantage of a process that is in-control but not in-spec?
Let's assume that at least some of the product
being made by this in-control process is in-spec.
Given that some of the process output is in-spec and the process is
in-control, we can actually statistically calculate how much
of the product will be out-of-spec.
Of course, the
goal is to have a process that is both in-control and
in-spec - that is what we call a capable and centered
process. What if you have a process that is
in-control, but not in-spec? By using SPC, root cause
analysis, and some common sense, an in-control process can
usually be brought in-spec.
How do you know
if your processes are in-control? A good first step to
take is to create a histogram of your data and then analyze
the pattern of variation to identify possible reasons for
the pattern. A control chart is really just an
extension of a histogram. Not only does it show you
the pattern of variation, but it plots that pattern over
time.
For more
information on in-control, in-spec, process capability,
histograms, patterns of variation, and control charts, please
visit our SPC Resource
Center.
Ray Mikulak
is president of Resource Engineering, Inc. You can
contact him at 800-810-8326 or 802-496-5888 or by e-mail. |
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Product
Focus from Carolyn Burke
SPC Web- and Computer-Based Training |
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 SPC
training can be time consuming, frustrating, and
dare we say sometimes even boring!
We have a solution: our comprehensive online training courses makes understanding
and applying statistical process control concepts
easy and interesting!
Our basic SPC course, SPC Workout, provides a solid
understanding of variation and the statistics upon
which control charts are based. The course covers
use of the most common variable and attributes
control charts and a conceptual overview of process
capability. SPC Workout has three units: Statistics
Primer, Using Control Charts, and Process Capability
Basics.
Our Advanced SPC course covers selecting and setting
up SPC control charts, determining the best control
chart format to use for a given process, what
patterns of instability tell you about a process, a
step-by-step tutorial for conducting process
capability studies and how to deal with process
capability complications. The Advanced SPC course
consists of two units: Advanced Control Charting and
Advanced Process Capability.
As with all of our programs, each unit contains
lessons to divide the content into manageable
learning segments. At the end of each unit, learners
have access to a Challenge to test their
comprehension of the body of knowledge covered in
the unit. Want to try out a free SPC training
lesson? Click here.
Carolyn
Burke is Sr. Customer Service Representative.
She can be reached toll free directly at
866-791-1003 or by e-mail. |
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Robin's Training Tip
What is the BEST Way
to Teach SPC to Operators? |
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 Most
manufacturing operators are hands-on people. That is
why they enjoy what they are doing. So, it is not
surprising that being scheduled for a classroom SPC course
is not typically met with great joy and enthusiasm from
front line folks. Not only are operators often turned
off by the thought of sitting in a classroom, but when the
training topic includes the "S" word, statistics that is :-),
it can be a double bummer.
Many companies
use our computer-based training in a classroom setting.
If you are doing that and it is working for you then by all
means, keep up the good work. However, if you are not happy with the results, we
encourage you to rethink your approach. Our training
was developed for individuals to go
through the training on their own, at their own pace.
Time and again
we get reports right from operators themselves that they
actually enjoyed our SPC Workout course. Better yet,
they tell us how much they have learned! How could
this be?
First, keep in
mind, operators are hand-on people. Our courses are
hands-on. The operator is the one running the
training. The operator decides the pace of the
training. The operator does the exercises and takes
the challenges. Basically, the operator is in control
on his/her own training.
Second,
classroom training is often conducted as full or half-day sessions.
Sitting at a desk for that long can drive some shop floor
folks crazy. They are used to being active. With
our web- and computer-based courses, training can be (and
should be) taken in shorter time chunks - say 45 to 60
minutes. And, because we cover the same amount of
content in 4 to 6 hours that is usually covered in a 2-day
SPC classroom session, our computer-based courses make
effective use of time.
So, our best
recommendation for basic SPC training is to use SPC Workout (of
course ;-) but have operators go through the course on their own.
This will give them the basic foundation that they need to
apply SPC in their jobs.
Robin McDermott
is director of training for Resource Engineering, Inc.
You can contact her at 800-810-8326 or 802-496-5888 or by
e-mail. |
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Some SPC Fun Facts |
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Dr.
Walter Shewhart developed the concept of
statistical control and control charts while
working at Bell Telephone Laboratories in the
1920s. |
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During World War II, control charts were used in
the United States to help improve the quality of
production of planes weapons to support the war
effort. However, after World War II ended, US
industry lost interest in the use of statistical
quality control. |
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The
“Western Electric Statistical Quality Control
Handbook,” arguably the bible on the application
and use of control charts, was originally
published in 1956. |
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Six
Sigma quality has been accepted to mean a
4.5-sigma process, not a “true six sigma”
process. A process that operates with “true six
sigma” performance takes up 50% of the
specification if centered. This gives it a Cpk
and a Cp of 2.0. A process such as this will
produce defects at a rate of only ~2 parts per
billion. Six Sigma professionals have allowed
for the process to drift by up to 1.5 standard
deviations from the mean. So if we have a
process with a Cp = 2.0 but allow for a 1.5s
drift, then we have the equivalent of a 4.5
sigma process. A 4.5 sigma process yields a 3.4
ppm defect level. |
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What's New?
Check out our new
Statistical Process Control Resource Center. It's
FREE and has lots of helpful
information for people just learning about SPC as well as some
interesting information that folks who have been using SPC for years
should find interesting.
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