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Accuracy & Precision
Two important points in measurement
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Accuracy and Precision
Objective:
Explain the difference between the accuracyand precision.
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Definitions
Accuracy and Precision
…sounds like the samething…
…is there adifference??
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Accuracy = how close a measured value is to the actual (true)value .
For example, we would expect a balance to read 100 grams if weplaced a standard 100 g weight on it. If it doesn’t, then the
balance is inaccurate.
Accura
cy
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Precision
Precision = how close themeasured values are to each
other .
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A precise instrument also gives a consistent reading when it is used repeatedly for thesame measurements.
A
B
A beaker is weighed on A, 3 times:
The readings are: 73 g, 77 g, 71 g
It is then weighed on B, 3 times:The readings are: 75 g, 73 g, 74 g
Balance B has better precision .Its readings are grouped closer together .
E.g.
Precision = how close a measurement is toother measurements of the same thing.
Which scale is more precise?
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Threetargets withthree
arrows eachto shoot.
Can you hit the bull's-eye?
Bothaccurateandprecise
Precisebut notaccurate
Notaccurateor precise
How dothey
compare?
Can you define accuracy vs. precision?
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Determining accuracy and precision
Time Scale 1 Scale 2 Scale 3
8 a.m. 110 pounds 124 pounds 142 pounds
Noon 113 pounds 126 pounds 103 pounds
4 p.m. 109 pounds 125 pounds 115 pounds
8 p.m. 110 pounds 126 pounds 129 pounds
Weight of person who weighs about 125 pounds
PRECISE, BUTNOT ACCURATE
ACCURATE ANDPRECISE
NOT ACCURATENOT PRECISE
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Learning Check: Accuracy
Who is more accurate when measuring a bookthat has a true length of 17.0 cm? ( Accuracy = how
close a measurement is to the correct or accepted value. )Susan :17.0 cm, 16.0 cm, 18.0 cm, 15.0 cm
Amy :15.5 cm, 15.0 cm, 15.2 cm, 15.3 cm
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Learning Check: Precision
Who is more precise when measuring the same17.0 cm book?
Susan :17.0 cm, 16.0 cm, 18.0 cm, 15.0 cm
Amy :15.5 cm, 15.0 cm, 15.2 cm, 15.3 cm
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LearningCheck: Precision
Precision = how close a measurement is to other measurements of the same thing .
Which set is more precise?18.2 , 18.4 , 18.35 - SET 117.9 , 18.3 , 18.85 - SET 2
16.8 , 17.2 , 19.44 - SET 3
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Accuracy vs. Precision
High AccuracyHigh Precision
High PrecisionLow Accuracy
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Graduated Cylinder – Meniscus andParallax
Beware of Paral lax – the apparent shift in position when viewed at a differentangle.
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Learning Check
Which set of measurements of a 2.00-gstandard weight is the most precise?
• A. 2.00 g, 2.01 g, 1.98 g• B. 2.10 g, 2.00 g, 2.20 g• C. 2.02 g, 2.03 g, 2.04 g• D. 1.50 g, 2.00 g, 2.50 g