The Standard Deviation is a statistical measure, that is used to identify the amount that a set of 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,
The Excel STDEV function calculates the sample standard deviation of a supplied set of values.
The syntax of the function is:
where the number arguments are one or more numerical values, arrays of values, or references to cells containing numbers.
If you are using a recent version of Excel (2007 or later), you can enter up to 255 number arguments to the function. However, in Excel 2003, the function can only accept up to 30 number arguments.
Note that the Stdev function is used when calculating the standard deviation for a sample of a population (e.g. if your data set records the individual heights of a sample of UK males). If you are calculating the standard deviation of an entire population, you should use the Stdevp or the Stdevpa function.
The Excel Stdeva function is very similar to the Excel Stdev function, in that both functions calculate the sample standard deviation of a supplied set of values.
The difference between these two functions applies when an array of values, containing text or logical values is supplied to the function. In this case, the Stdev function ignores the text and logical values, whereas the Stdeva function assigns the value 0 to text and the values 1 or 0 to logical values.
The rules for this are shown in the table below, compared to the rules for the Stdev function.
STDEV Function  STDEVA Function  

Logical values or text (including text representations of numbers), typed directly into the list of arguments  ARE counted (TRUE=1, FALSE=0)  ARE counted (TRUE=1, FALSE=0) 
Text that cannot be interpreted as a number, typed directly into the list of arguments  #VALUE! error  #VALUE! error 
Logical values, within arrays or reference arguments  Ignored  ARE counted (TRUE=1, FALSE=0) 
Text (including empty text "", text representations of numbers, or other text), within arrays or reference arguments  Ignored  Counted as zero 
Empty cells  Ignored  Ignored 
See the page on Standard Deviation in Excel for a comparison of all the different Excel standard deviation functions.
The following spreadsheet shows the Excel Stdeva function used to calculate the standard deviation of the set of values in cells A1A4 and in cells A1A6.
Formulas:
 Results:

Note that, in the above spreadsheet, the function in cell B2 includes the values in cells A5 and A6. In this case, the logical value TRUE in cell A5 is treated as the value 1 and the text in cell A6 is treated as the value 0. This is shown by the example in cell B3, in which cells A1A4 and the values 1 and 0 give the same result as the standard deviation calculation for cells A1A6.
For further information and examples of the Excel Stdeva function, see the Microsoft Office website.
If you get an error from the Excel Stdeva function this is likely to be one of the following:
#DIV/0!    Occurs if fewer than two numeric values are supplied to the Stdeva function. 
#VALUE!    Occurs if any values that are supplied directly to the Stdeva function are text values that cannot be interpreted as numbers. 