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STDEV Function
Standard Deviation

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.

For any set of values, the Standard Deviation is give by the square root of the variance. ie.

where x takes on each value in the set, x is the average (statistical mean) of the set of values, and n is the number of values in the set.

A full explanation can be found on the Wikipedia Standard Deviation page
STDEVP & STDEV.P Functions

In Excel 2010, the STDEVP function has been replaced by the STDEV.P function, which has improved accuracy.

Although it has been replaced, the Stdevp function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.

The Excel STDEVP function calculates the standard deviation of a supplied set of values.

The format of the function is :

STDEVP( number1, [number2], ... )

where the arguments, number1, [number2], etc, are one or more numerical values or references to cells containing numbers.

If you are using Excel 2007 or Excel 2010, 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 Stdevp function is used when calculating the standard deviation for an entire population.
If you are calculating the standard deviation of an __sample__ population, you
need to use the Stdev function.

Note also, that the Stdevp 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 Stdevpa function.

A company keeps a record of its monthly sales figures, over the last three years. These are stored in cells B3 - B14, D3 - D14 and F3 - F14 of the spreadsheet on the right.

The standard deviation of the three years' sales figures is calculated in cell H3 of the spreadsheet. The formula for this, as shown in the formula bar, is :

=STDEVP( B3:B14, D3:D14, F3:F14 )

As shown in cell H3, the standard deviation for the 3 years of sales figures is **2,484.05**

In the example above, the arguments to the Stdevp function are input as 3 cell ranges. However, you can also input figures directly, as individual numbers or number arrays.

For example, if, during January and February 2010 the sales figures are 13,000 and 14,500, you could add these directly into the above function as follows:

Either as individual numbers:

=STDEVP( B3:B14, D3:D14, F3:F14, 13000, 14500 )

Or, as an array of numbers:

=STDEVP( B3:B14, D3:D14, F3:F14, {13000, 14500} )

This gives the updated standard deviation value of **2,435.35**

Further information and examples of the Excel Stdevp function can be found on the Microsoft Office website.

If you get an error from the Excel Stdevp function this is likely to be one of the following:

Common Errors

#DIV/0! | - |
Occurs if none of the values supplied to the function are numeric (Note that text representations of numbers, that are supplied as a part of an array, are not interpreted as numeric values by the Stdevp function) |

#VALUE! | - | Occurs if any values that are supplied directly to the Stdevpa function are text values that cannot be interpreted as numeric values. |