Barringer & Associates,
Inc.,
ACCURACY AND PRECISION
Look over the shoulder of hunters as they take their trusty rifle to
the local shooting range during the weeks before they commence their annual
pilgrimage as deer hunter. You’ll
quickly observe the art of hitting the bulls-eye of a target is not an
inherited trait passed along through the genes of modern man. Aspiring Nimrods take a shot or two and then
adjust their rifle shots as they scatter lead over the target and miss the
bulls-eye. They foolishly chase unknown
problems until their shoulders are raw and their wallets empty while blaming
their problems on the rifle, ammunition, wind, etc. Amateurs can’t fix their problems of accuracy
and precision as can marksmen:
Accuracy is how close you group your shots to the center of the bulls-eye. Accuracy is conformity to the true value of
the standard. Accuracy avoids bias with
calibration or adjustment corrections.
Precision is how close you group your shots together. Precision measures scatter among
measurements. Precision is the small
random error of individual attempts, which are uncorrectable by calibrations or
adjustments.
Repeatability is
how well you repeat the results each time with the same rifle, ammunition,
etc. Repeatability is closeness of
agreement between successive results using the same method, same operator, same
materials, etc.
Reproducibility is how well another person uses your rifle, ammunition, etc. Reproducibility is similar to repeatability
using identical materials but under different conditions such as different
operators, different times, etc.
Wise Nimrods make three to five shots before they analyze their
problems. If the problem involves
accuracy, they adjust sights on the rifle.
If the problems involves precision, they don’t adjust sights but work on
the assignable problems within the process of shooting their rifles. Then they repeat the study sequence with
three to five more shots followed by specific revisions for fixing specific
problems. Wise Nimrods separate chance
events involving precision from assignable causes involving accuracy. When they have solved the correct problems,
then they work on repeatability in their shooting. You can’t adjust rifle sights to correct
precision problems. Knowing differences between accuracy and precision lets you fix
specific problems correctly the first time.
Machine operators have the same problems as aspiring Nimrods. They must understand the separate concepts of
accuracy and precision so they make machine tool adjustments correctly. We calibrate (reduce the bias) measuring
instruments as we make accuracy corrections.
However, inherent precision (scatter) of the measuring instrument is
built into the measuring process which involves the person, procedure,
instrument, etc. Knowing the differences
between accuracy and precision helps speed set-ups, reduces times lost for
unnecessary adjustments, and reduces scrap by not hitting the bull’s-eye of
conformance. Precision problems arise from randomness in every measurement and every
manufacturing activity. Accuracy
problems arise from errors in the aim point.
The randomness of precision problems causes confusion as operators ask:
Is this an accuracy problem? Is this a
precision problem? These questions delay
making the correct decision about altering the process to control the machine
to achieve conformance. Solving the
precision problem requires control of many small errors to make the scatter
small. Solving the accuracy problems
requires knowing the aim point and adjusting the aim point so that it is near
the mid point of the tolerance. The
bull’s-eye for manufacturing is the middle ¼ of the tolerance zone. The key to success is simple. First,
aim for the bull’s-eye. Second, control
scatter in the measurements. It is
easy to say but hard to do. Compare
target examples below to understand the difference between accuracy and
precision.
Accuracy
and Precision is: Setting
priorities for problem solving and understanding the scatter in
measurements. Accuracy and precision
separates a craftsman’s skill from the amateurs excuses. Nimrods and machine operators keep their
shots in the bulls-eye. Hitting the
bulls-eye every time provides jobs and security for you and me.
Precise
but not accurate Accurate
but not precise Accurate and
precise Accurate and
very precise
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© Barringer & Associates, Inc. 1995