standard error (s.e.)

The variability of the mean is called the standard error of the mean (or just s.e.), and is often drawn as little ‘error bars’ on graphs or histograms, to give you some idea of the accuracy of the average measure. If the mean is averaged over a sample of N independent items, the standard error is σ/√N – where σ is the standard deviation of the sample or the standard deviation of a single item if that is already known.