The Excel AVERAGEIF function finds values in a supplied array, that satisfy a given criteria, and returns the average (ie. 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 given criteria 
criteria    The condition to be tested against each of the values in range 
[average_range]   
An optional array of numeric values (or cells containing numbers) for which the average is to be calculated, if the corresponding range entry satisfies the supplied criteria. If the average_range argument is omitted, the values in the initial range argument are averaged. 
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.
eg. 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:
The Excel Averageif function is not casesensitive. So, for example, the text strings "TEXT" and "text" will be classed as equal.
The following spreadsheet shows several examples of the Excel Averageif function.
Formulas:

Results:

In the above spreadsheet, each call to the Excel Averageif function tests the contents of cells A1 to A14 against the supplied criteria. The average calculation uses the corresponding cells in the range B1 to B14.
The formula in cell A16, for example, requires the average of cells in the range B1  B14, for which the corresponding values in column A are equal to the value "Thursday". As cells A3 and A5 are both equal to the text string "Thursday", the average is calculated for the values in cells B3 and B5.
Further examples of the Excel Averageif function can be found on the Microsoft Office website.
The most common errors from the Excel Averageif function are listed in the table below :
#DIV/0!   
Occurs if either:
none of the values in the supplied range array satisfy the supplied criteria orthe values to be averaged are all nonnumeric 
#VALUE!    Occurs if the criteria argument is a text string with length greater than 255 characters. 