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Sources of Uncertainty
Whenever you measure or observe values, a degree of uncertainty is associated with it. Common sources of uncertainties include: human error; failure to account for all possibilities in an experiment; failure to properly calibrate an instrument; variations in instrument operation due to weather, time of day or environmental factors; and differences in the operations of the same type of instrument due to manufacturing or care inconsistencies. Often, experimental results are displayed with a plus-or-minus symbol next to them. This value gives you the range of results seen.
Types of Errors
Two main types of errors affect uncertainty experiments. Random errors are caused by unpredictable events that affect results. Random errors can be averaged over a large number of experiments and provide an accurate plus-or-minus value. The other type of error, the systematic error, can be harder to analyze. This error usually is caused by a consistent limitation of the instrument or the experimenter, which provides a similar but incorrect result.
Accuracy
An experiment's uncertainties refer to two specific concepts: its accuracy and its precision. Accuracy is a measure of how closely a result agrees with or is in the range of an accepted value. Accepted values usually have been established through averaging the observed results of prior, similar experiments. The accuracy depends on the differences, if any, between the expected result and the actual result.
Precision
Precision goes hand-in-hand with accuracy, measuring how reproducible the results of the uncertain experiment are. Precision describes the consistency or reliability of the measurements in a single experiment, compared with measurements from similar, prior experiments. Accuracy drives precision, but precision does not always include accuracy.