The mean deviation for any distribution is always 0 and so conveys no information whatsoever.
The standard deviation is the square root of the variance. The variance of a set of values is the sum of the probability of each value multiplied by the square of its difference from the mean for the set.
A simpler way to calculate the variance is
Expected value of squares - Square of Expected value.
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Standard deviation is the variance from the mean of the data.
The mean is the average value and the standard deviation is the variation from the mean value.
The mean deviation (also called the mean absolute deviation) is the mean of the absolute deviations of a set of data about the data's mean. The standard deviation sigma of a probability distribution is defined as the square root of the variance sigma^2,
No, since "between 3" doesn't mean anything. Do you mean between -3 and +3?
no the standard deviation is not equal to mean of absolute distance from the mean