Accuracy and Precision

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Accuracy and Precision
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40
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• A MEASURE
• of your
• SUCCESS!

2
ITS TRUE!

• Accuracy
• indicator of the closeness of a measurement
  to its true or accepted value
• Dependent upon
• how carefully the measurement is taken
• nature of the instrumentation used

3
Right On!

• Precision
• describes the agreement among several results
  measured in the same way
• reproducibility of measurements

4
Tell Me!
High or Low accuracy and precision
High accuracy High precision
Low accuracy Low precision
Low accuracy High precision
5
Lab things to consider

• Procedures v. Data table
• Using the balance
• Volume v. volume
• What is a meniscus?
• Collecting data v. analyzing data
• Does everyone have a calculator?
• Are all those zeros necessary?

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Significant figures (or digits)

• Things to consider
• D m/V m 14.3216 g V 2.00 cm3
• D ?
• Exact numbers (no uncertain digit)
• Counted numbers (students in the class)
• Numerical relationships (1 km 1000 m)
• Inexact numbers (last digit is always uncertain
  digit)
• Result from measurement ( 25.3 mL in grad.
  Cylinder)
• Depend upon what measuring device is being used

• Uncertain digit
• Last digit in a measurement that can be read with
  reasonable reliability
• Deviation Page 33

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Significant Figures

• RULES TO LIVE BY
• All nonzero digits are significant

• One or more final zeros used after the decimal
  point are always significant

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Significant Figures

• Zeros between other significant digits are
  always significant

• Zeros used solely for spacing the decimal point
  are not significant. (place holders)

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Calculating with Sig. Figs.
1. Addition and Subtraction the answer must
be rounded to the same number of digits to the
right of the decimal point as there are in the
measurement with the smallest number of digits to
the right of the decimal
EX. 2.89 m 0.0043 m 2.8943 m 2.89 m
(correct of sig figs)
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Calculating with Sig. Figs.

• 2. Multiplication and Division
• the product or quotient should be rounded to
  the same number of significant figures as in the
  measurement with the fewest total number of
  significant figures

Ex. 3.5293 g x 34.2 g 120.70206 g2
121 g2 (correct of sig figs)