The Excel AVERAGEIF function finds the values in a supplied array that satisfy a specified criteria, and returns the average (i.e. the statistical mean) of the corresponding values in a second supplied array.
The function is new in Excel 2007, and so is not available in earlier versions of Excel.
The syntax of the Averageif function is:
Where the function arguments are:
range    An array of values (or range of cells containing values) to be tested against the supplied criteria. 
criteria    The condition to be tested against each of the values in the supplied range. 
[average_range]   
An optional array of numeric values (or cells containing numbers) for which the average is to be calculated, if the corresponding value in range satisfies the supplied criteria. If the [average_range] argument is omitted, the average is calculated for values in the initial supplied range. 
Wildcards
You can also use the following wildcards in textrelated criteria:
?  matches any single character
*  matches any sequence of characters
if you do actually want to find the ? or * character, type the ~ symbol before this character in your search.
E.g. the condition "A*e" will match all cells containing a text string beginning with "A" and ending in "e".The supplied criteria can be either:
Note also that the Excel Averageif function is not casesensitive. So, for example, the text strings "TEXT" and "text" will be evaluated as equal.
Cells A16A20 of the following spreadsheet shows five examples of the Excel Averageif function.
For each call to the Excel Averageif function the range argument (to be tested against the criteria) is the cell range A1A14 and the [average_range] argument (containing the values to be averaged) is the cell range B1B14.
Formulas:

Results:

Note that, in cells A16, A18 and A20 of the above 'Formulas' spreadsheet, the text value "Thursday" and the expressions ">2" and "<>TRUE" are encased in quotes. This is essential for all text or expressions.
For further examples of the Excel Averageif function, see the Microsoft Office website.
If you get an error from the Excel Averageif function, this is likely to be one of the following:
#DIV/0!   
Occurs if either:

#VALUE!    Occurs if the criteria argument is a text string that is greater than 255 characters in length. 