Related Function:
The STDEV.P function returns the standard deviation of a supplied set of values.
 This function was introduced in Excel 2010 and so is not available in earlier versions.
 The STDEV.P function replaces the STDEVP function included in earlier versions of Excel.
Syntax
=STDEV.P(num_1,[num_2], … )
Arguments
Argument  Description 

num_1  The first number, cell references or range argument corresponding to a population 
[num_2], …  Optional. Additional numbers, cell references or ranges for which you want the skew, up to a maximum of 255 
Examples
A  B  C  D  

1  Data  
2  10,647  
3  10,677  
4  17,193  
5  14,162  
6  11,609  
7  17,381  
8  10,292  
9  
10  Formula  Result  Notes  
11  =STDEV.P(A2:A8)  2,883.297  Standard deviation of values provided in cells A2:A8 
Common Function Error(s)
Problem  What went wrong  

#VALUE!  Occurs if any values that are supplied directly to the function are text values that cannot be interpreted as numeric values  
#NUM!  Occurs if none of the values that are supplied to the function are numeric

Standard deviation is a statistical measure, that is closely linked to the variance. Both measures are commonly used across a set of values, to identify the amount the values differ, or deviate, from the average value.
Standard deviation is give by the square root of the variance, i.e.:
where,
 x is the sample mean
 n is the sample size
See Wikipedia for more information on standard deviation.