The mean's standard error turns out to equal the population standard deviation divided by the square root of the sample size, and is estimated by using the sample standard deviation divided by the square root of the sample size. The sample mean's standard error is the standard deviation of the set of means that would be found by drawing an infinite number of repeated samples from the population and computing a mean for each sample. The standard deviation of a population or sample and the standard error of a statistic (e.g., of the sample mean) are quite different, but related. A useful property of the standard deviation is that, unlike the variance, it is expressed in the same unit as the data. It is algebraically simpler, though in practice less robust, than the average absolute deviation. The standard deviation of a random variable, sample, statistical population, data set, or probability distribution is the square root of its variance. Standard deviation may be abbreviated SD, and is most commonly represented in mathematical texts and equations by the lower case Greek letter sigma σ, for the population standard deviation, or the Latin letter s, for the sample standard deviation. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, while a high standard deviation indicates that the values are spread out over a wider range. In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values.
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