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:
  a numeric value (including integers, decimals, dates, times, and logical values) (eg. 10, 01/01/2008, TRUE)  
or  
  a text string (eg. "Text", "Thursday")  MUST be supplied in quotes  
or  
  an expression (eg. ">12", "<>0")  MUST be supplied in quotes 
and can be entered into the function either directly, as a value returned from another function, or as a reference to a cell containing a value.
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.
The spreadsheet on the left shows the formulas used and the spreadsheet on the right shows the results.
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. For of the cells that satisfy the criteria, the corresponding cells in the range B1 to B14 are included in the average calculation.
In cell A16, for example, the cells A3 and A5 are both equal to the text string "Thursday". Therefore, the average is calculated for the corresponding values from cells B3 and B5 (100 and 200), resulting in an average value of 150.
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:


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