Slide 1
"If you have only one
watch, you always
know exactly what
time it is. If you have
two watches, you are
never quite sure..."
Slide 2
When we take a single reading
we tend to accept the readout
without really thinking about its
validity.
People seem to do this even
when they know the reading is
inaccurate.
Your digital watch is probably off
by a minute or two right now!
Slide 3
Every measurement is subject
to errors and inaccuracies.
Modern digital instruments
convey such an aura of
accuracy and reliability, that we
forget this basic rule...
There is no such thing as
a Perfect Measurement
Slide 4
Talking about Measurement
The result obtained from counting
will be an exact result (barring
blunder)
The result obtained from
measuring will only approach the
truth
Slide 5
Talking about Measurement
Precision
Accuracy
Resolution
Slide 6
Talking about Measurement
“Accuracy is telling the
truth . . . Precision is
telling the same story
over and over again.”
-Yiding Wang
Slide 7
Accuracy
A measurement is accurate if it correctly
reflects the size of the thing being
measured.
Accurate means "capable of providing a
correct reading or measurement." In
physical science it means 'correct'.
The degree of conformity with a
standard. Accuracy relates to the quality
of a result.
Slide 8
Precision
Precision refers to the reproducibility of a
result or measurement.
In physical science it means "repeatable,
reliable, getting the same measurement
each time."
The degree of refinement in the
performance of an operation, or the
degree of perfection in the instruments
and methods used to obtain a result.
Slide 9
Talking about Measurement
To understand the difference between
these two properties, let us use the
analogy of the marksman who uses a
gun to fire bullets at a target. In this
analogy, the gun is the instrument, the
marksman is the operator of the
instrument, and the results are
determined by the location of the bullet
holes in the target.
Slide 10
In this figure, we see that the
marksman's instrument was
quite precise, since his results
were uniform.
The instrument did not provide
accurate results since the shots
were not centered on the
target's bull's eye.
The fact that his results were
precise, but not accurate, could
be due to a consistent operator
error.
Therefore precision tells us
something about the quality of
the instrument's operation.
Precision
Slide 11
In this figure, we see the shots
are centered on the bull's eye
but the results were not uniform.
This indicates that the
marksman's instrument
displayed good accuracy but
poor precision.
In this case, the marksman will
never achieve both accuracy
and precision. If he is not
satisfied with the results he
must change his equipment.
Therefore accuracy tells us
something about the quality or
correctness of the result.
Accuracy
Slide 12
Accuracy & Precision
As scientists, we desire our
results to be both precise
and accurate.
As shown in this figure, the
shots are all uniform and
centered on the bull's eye.
This differs from the first
figure in that the marksman
has compensated for the
error.
Slide 13
Accuracy & Precision
Slide 14
Slide 15
Slide 16
Slide 17
Slide 18
Resolution
Resolution is the ability of the
measurement system to detect
and faithfully indicate small
changes in the characteristic of
the measurement result
Slide 19
Resolution
The smallest interval that can
be measured.
Tools with smaller resolution
will generally have greater
accuracy.