measurement by cuiliqing


									                          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

People seem to do this even
when they know the reading is

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

The result obtained from
measuring will only approach the
                            Slide 5

Talking about Measurement

                             Slide 6

 Talking about Measurement

“Accuracy is telling the
  truth . . . Precision is
 telling the same story
 over and over again.”
       -Yiding Wang
                                        Slide 7

A measurement is accurate if it correctly
   reflects the size of the thing being

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 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          Precision
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
Therefore precision tells us
something about the quality of
the instrument's operation.
                                           Slide 11

In this figure, we see the shots
are centered on the bull's eye      Accuracy
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.
                               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
                       Slide 13

Accuracy & Precision
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                                Slide 18


 Resolution is the ability of the
measurement system to detect
  and faithfully indicate small
changes in the characteristic of
   the measurement result
                                 Slide 19


The smallest interval that can
      be measured.

Tools with smaller resolution
 will generally have greater

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