Slide 1

"If you have only "If you have only one watch, you one watch, you always know always know

exactly what time it exactly what time it is. If you have two is. If you have two watches, you are watches, you are

never quite sure..."never quite sure..."

Slide 2

When we take a single reading When we take a single reading we tend to accept the readout we tend to accept the readout without really thinking about its without really thinking about its validity. validity.

People seem to do this even People seem to do this even when they when they knowknow the reading is the reading is inaccurate. inaccurate.

Your digital watch is probably off Your digital watch is probably off by a minute or two right now!by a minute or two right now!

Slide 3

Every measurement is subject Every measurement is subject to errors and inaccuracies. to errors and inaccuracies.

Modern digital instruments Modern digital instruments convey such an aura of convey such an aura of

accuracy and reliability, that we accuracy and reliability, that we forget this basic rule...forget this basic rule...

There is no such thing as a Perfect Measurement

Slide 4

Talking about MeasurementTalking about Measurement

The result obtained from counting The result obtained from counting will be an exact result (barring will be an exact result (barring blunder)blunder)

The result obtained from The result obtained from measuring will only approach the measuring will only approach the truth truth

Slide 5

Talking about Measurement Talking about Measurement

PrecisionPrecision

AccuracyAccuracy

ResolutionResolution

Slide 6

Talking about MeasurementTalking about Measurement

“Accuracy is telling the truth . . . Precision is telling the same story over and over again.”

-Yiding Wang

Slide 7

AccuracyAccuracyA measurement is accurate if it correctly A measurement is accurate if it correctly

reflects the size of the thing being reflects the size of the thing being measured.measured.

Accurate means "capable of providing a Accurate means "capable of providing a correct reading or measurement." In correct reading or measurement." In physical science it means 'correct'.physical science it means 'correct'.

The degree of conformity with a The degree of conformity with a standard. Accuracy relates to the quality standard. Accuracy relates to the quality

of a result.of a result.

Slide 8

PrecisionPrecision

Precision refers to the reproducibility of a Precision refers to the reproducibility of a result or measurement. result or measurement.

In physical science it means "repeatable, In physical science it means "repeatable, reliable, getting the same measurement reliable, getting the same measurement

each time." each time."

The degree of refinement in the The degree of refinement in the performance of an operation, or the performance of an operation, or the

degree of perfection in the instruments degree of perfection in the instruments and methods used to obtain a result. and methods used to obtain a result.

Slide 9

Talking about MeasurementTalking about Measurement

To understand the difference between To understand the difference between these two properties, let us use the these two properties, let us use the

analogy of the marksman who uses a analogy of the marksman who uses a gun to fire bullets at a target. In this gun to fire bullets at a target. In this

analogy, the gun is the instrument, the analogy, the gun is the instrument, the marksman is the operator of the marksman is the operator of the instrument, and the results are instrument, and the results are

determined by the location of the bullet determined by the location of the bullet holes in the target. holes in the target.

Slide 10

PrecisionPrecisionIn this figure, we see that the In this figure, we see that the marksman's instrument was marksman's instrument was quite precise, since his results quite precise, since his results were uniform.were uniform.

The instrument did not provide The instrument did not provide accurate results since the shots accurate results since the shots were not centered on the were not centered on the target's bull's eye.target's bull's eye.

The fact that his results were The fact that his results were precise, but not accurate, could precise, but not accurate, could be due to a consistent operator be due to a consistent operator error. error.

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

Slide 11

AccuracyAccuracyIn this figure, we see the shots In this figure, we see the shots are centered on the bull's eye are centered on the bull's eye but the results were not uniform.but the results were not uniform.

This indicates that the This indicates that the marksman's instrument marksman's instrument displayed good accuracy but displayed good accuracy but poor precision.poor precision.

In this case, the marksman will In this case, the marksman will never achieve both accuracy never achieve both accuracy and precision. If he is not and precision. If he is not satisfied with the results he satisfied with the results he must change his equipment. must change his equipment.

Therefore accuracy tells us Therefore accuracy tells us something about the quality or something about the quality or correctness of the result.correctness of the result.

Slide 12

Accuracy & PrecisionAccuracy & Precision

As scientists, we desire our As scientists, we desire our results to be both precise results to be both precise and accurate.and accurate.

As shown in this figure, the As shown in this figure, the shots are all uniform and shots are all uniform and centered on the bull's eye. centered on the bull's eye.

This differs from the first This differs from the first figure in that the marksman figure in that the marksman has compensated for the has compensated for the error. error.

Slide 13

Accuracy & PrecisionAccuracy & Precision

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ResolutionResolution

Resolution is the ability of the Resolution is the ability of the measurement system to detect measurement system to detect

and faithfully indicate small and faithfully indicate small changes in the characteristic of changes in the characteristic of

the measurement result the measurement result

Slide 19

ResolutionResolution

The smallest interval that can The smallest interval that can be measured. be measured.

Tools with smaller resolution Tools with smaller resolution will generally have greater will generally have greater

accuracy.accuracy.