Accuracy

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Accuracy Precision
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Accuracy

• Refers to how close a measured value is to the
  accepted or true value.

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Precision

• Refers to how close a series of measurements are
  to each other.
• Precise measurements show little variation over a
  series of trials. That means, theres not much
  scatter in the data.
• Precise measurements are not necessarily accurate!

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a)
c)
?
?
?
Not Accurate Not Precise
Accurate and Precise!
?
?
?
b)
d)
?
?
Precise but not Accurate
Accurate but Not Precise
?
?
?
?
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Now lets look at some data.
These students were asked to determine the
density of sucrose. Sucrose has a density of
1.59 g/cm3.
Student A (g/cm3) Student B (g/cm3) Student C (g/cm3)
Trial 1 1.54 1.40 1.70
Trial 2 1.60 1.68 1.69
Trial 3 1.57 1.45 1.71
Avg. 1.57 1.51 1.70
Range 0.06 0.28 0.02
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• Which student was most accurate?
• Which student was most precise?

Student A
Average value closest to accepted value.
Student C
Smallest range (high minus low value).
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Lets summarize

• Accuracy refers to
• Precision refers to

how close your data is to the true or accepted
value.
how close your data points are to each other.
Data that has a lot of scatter is not precise.