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Sample variance and standard deviation
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The sample variance is the mean of the square of the deviations of each measurement from the mean (called mean square deviation).

Enter xi values: (hit Autofill and Plot) to see the sample variance s2 and sample standard deviation s. 

The sum of deviations (xi - )= 0 is expected, since it follows that 
xi = N, i.e.  .

which is the definition of .

Most points (approx 2/3) lie within s of the mean .

N must be at least 2, since we cannot estimate a spread in values from a single measurement.

s is an estimate of s, the standard deviation of a single measurement. It is not an estimate of s(), the standard deviation of the mean ... next.


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