Sunday, October 31, 2010

Accuracy and Precision

  • No measurement is exact
  • all estimates are only best estimates, which have some degree of uncertainty
  • you can only get an exact number when you're counting objects
Absolute Uncertainty, the uncertainty expressed in the units of measurement, not as a ratio

Method 1

  1. Make at least 3 measurements
  2. Calculate the average
  3. The absolute uncertainty= the largest difference between the average and the lowest/ highest reasonable measurement
Method 2 (determining the uncertainty of an instrument)

  1. When making a measurement, always measure to the best precision that is possible
  2. You should estimate the measurement to a fraction of 0.1 of the smallest segment on the instuments scale
  3. On a ruler, the smallest segment is 1mm. The best precision should be to break this into 10 equal pieces. (0.1mm)
Relative Uncertainty and Sigfigs

Relative uncertainty = absolute uncertainty
  • Can be expressed in percent (%)
  • Or expressed using sigfigs
  • The number of sigfigs indicates the relative uncertainty
Try some practice questions:

Find the absolute uncertainty:

Trial #                          Mass of juice boxes  

1                                  200.35
2                                  200.34
3                                  200.34
4                                  200.38
5                                  200.33

Trial#                          Mass of a water jug

1                                 500.29
2                                 500.30
3                                 500.32
4                                 500.24
5                                 500.30


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