The STDEV.S function calculates the sample standard deviation of a supplied set of values.
 This function is new in Excel 2010 and so is not available in earlier versions of Excel. However, the STDEV.S function is simply a new version of the STDEV function that is available in earlier versions of Excel.
Syntax
=STDEV.S(num_1,[num_2], …)
Arguments
Argument  Description 

num_1  The first number argument corresponding to a sample of a population 
[num_2], …  Optional. Additional number arguments 2 to 254 corresponding to a sample of a population 
Note: Arguments to the STDEV.S function can be input as cell ranges, individual numbers, or number arrays, e.g., in the table below:
 The result in J3 uses a range of cells (see K3)
 The result in J4 uses cell ranges plus individual numbers (see K4)
 The result in J5 uses cell ranges plus a number array (see K5)
Examples
A  B  C  D  E  F  G  H  I  J  K  

1  2010  2011  2012  2013  Standard Deviation: 

2  Month  Amount  Month  Amount  Month  Amount  Month  Amount  Formula used  
3  Jan10  15,000  Jan11  17,500  Jan12  13,000  Jan13  13,000  2,519.29  =STDEV.S(B3:B14,D3:D14,F3:F14)  
4  Feb10  14,500  Feb11  12,000  Feb12  15,000  Feb13  14,500  2,468.04  =STDEV.S(B3:B14,D3:D14,F3:F14,13000,14500)  
5  Mar10  14,500  Mar11  16,000  Mar12  14,000  2,468.04  =STDEV.S(B3:B14,D3:D14,F3:F14,{13000,14500})  
6  Apr10  14,000  Apr11  19,000  Apr12  16,500  
7  May10  16,000  May11  17,000  May12  20,000  
8  Jun10  9,500  Jun11  10,500  Jun12  12,500  
9  Jul10  13,500  Jul11  11,000  Jul12  14,000  
10  Aug10  17,000  Aug11  12,500  Aug12  18,500  
11  Sep10  11,000  Sep11  13,000  Sep12  14,500  
12  Oct10  15,000  Oct11  15,500  Oct12  13,000  
13  Nov10  17,500  Nov11  15,000  Nov12  13,000  
14  Dec10  18,000  Dec11  17,500  Dec12  17,000 
Usage note:
 The STDEV.S function is used when calculating the standard deviation for a sample of a population. If you are calculating the standard deviation of an entire population, you need to use the STDEV.P function.
 The STDEV.S function ignores text values and logical values if these are supplied as part of an array. If you want a function that includes text and logical values in the calculation, consider using the STDEVA function.
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  
#DIV/0!  Occurs if fewer than two numeric values are supplied to the function

The 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 that the values differ, or deviate, from the average value.
When your data set is a sample of a population, rather than an entire population, you should use a slightly modified form of the Standard Deviation, known as the Sample Standard Deviation. The equation for this is:
where,
 x takes on each value in the set
 x is the average, statistical mean, of the set of values
 n is the number of values.
A full explanation can be found on the Wikipedia Standard Deviation page
.