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It is used, along with significant digits, to indicate the precision of a measurement. Thus, in the two-significant-digit measurement 5.4 0.2 meters, the uncertainty interval is 0.2 meters. This means that the true value of the measurement is believed to lie between 5.2 meters and 5.6 meters.
Often, the uncertainty of a measurement is found by repeating the measurement enough times to get a good estimate of the standard deviation of the values. Then, any single value has an uncertainty equal to the standard deviation. However, if the values are averaged, then the mean measurement value has a much smaller uncertainty, equal to the standard error of the mean, which is the standard deviation divided by the square root of the number of measurements.