Two sets of data could have the same mean but have a very different spread of data. Standard Deviation will tell you the extent of this - in other words how reliable the mean is. A low standard deviation indicates that the data points tend to be very close to the mean, whereas high standard deviation indicates that the data is spread out over a large range of values and the mean is less reliable as there is obviously a lot of variation in the sample.
Standard deviation is very useful when used to compare the dispersion of two or more sets of data. The standard deviation links the data set to normal distribution. In a normal distribution:
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