Absolute Uncertainty
Within every set of data there is an absolute uncertainty. This is the average of the numbers minus the number with the largest difference. Before you can caculate the average though you need to make sure to remove all inprecise data.
For example the numbers 11.9cm, 12cm, 11.8cm, 10.6cm, 11.7cm
You would remove the 10.6cm because it does not match the other data.
Next you would find the average by adding all the numbers that are left then dividing giving you 11.85
Then you would find the number with the largest difference which would be 11.7 and this would give you the answer 11.85 +/- 0.15 cm
Also this can be represented as a percentage by dividing the uncertain value by your average
The lower the percentage the more precise the values were.
Uncertainties in Measuring
When using a measuring tool you can estimate one more decimal place than the actual value shown on the the tool. Like on a ruler that is in mm you can guess one more decimal place further than mm. This value is one tenth of the smallest unit of measuring on a tool. With a 50mL graduated cyclinder your uncertainty would be 5mL.
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