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


-Lauren