Uncertainty

 Terms you should know and understand

Accuracy – how close the measurements are to the “true” value.

Precision – how close are the repetitive measurements across each trial. How repeatable the measurements are.

Uncertainty – provides an indication on the reliability of the measurement. Indicates how far from the true value the measurement is and/or how good of a tool you used to measure.

Use of Uncertainty –

For Biology, the uncertainties are the smallest division of the instrument used to do the measuring. “The accepted rule is that the degree of precision is ± the smallest division on the instrument. For example, a leaf measured using a ruler whose smallest division is 1 mm would measure 4.5 cm ± 0.1 cm.” – S. Sulzberger, IB Biology Moderator, Posting on 10/03/2005.

Examples:

The smallest unit of measure on the classroom clock is 1 second. Raw data collected using this clock must be measured in seconds ± 1s. This value is part of the units required on the equipment list, in raw data and as appropriate for processed data.

If you are recording beats per minute there is also an uncertainty due to the ability to record the beats per minute. If you measure the number of beats every 15 seconds the value could be 20 beats every 15 seconds ± 1 beat. If they want to indicate the number of beats every 60 seconds, the value would be 4 times the value recorded every 15 seconds ±  4 seconds.

Chemistry and Physics have a different requirement, which could include calculating the uncertainty, or more commonly reporting half the smallest division on the scale. The last digit of any number is generally considered a guess. You may use this method for your uncertainty – just be certain you are consistent.

Example:

The smallest unit of measure on the classroom clock is 1 second. For raw data collected using this clock the values would be measured in seconds ± 0.5s.

Some equipment has the uncertainty indicated on the equipment. Graduated cylinders are a good example. Our 50mL graduated cylinders generally have an uncertainty of ± 1mL.

Uncertainty is to be indicated with the value and/or equipment it describes. For materials, any measuring device must have the units of measure AND the level of uncertainty. Uncertainty must be indicated on all measured raw data. For data tables one can put the level of uncertainty at the top of the column. For processed data, the uncertainty should be carried through to the graphical representation of the data. This is particularly important for axis labels such as “Time in seconds ±1s.”

In Biology, we do not require the students undergo error propagation, which is required in Chemistry and Physics.