In Excel 2010, the TTEST function has been renamed T.TEST.
Although the Ttest function has been replaced, Excel 2010 has kept this function, (stored within the list of compatibility functions), for compatibility with earlier versions of Excel.
The Excel TTEST function calculates the probability associated with the Student's T Test, which is commonly used for identifying whether two data sets are likely to have come from the same two underlying populations with the same mean.
The format of the function is :
where the function arguments are:
array1    The first data set  
array2    The second data set (must have the same length as array1)  
tails   
The number of tails for the distribution. This must be either:


type   
An integer that represents the type of ttest. This can be either:

A  B  

1  4  8 
2  5  3 
3  2  7 
4  5  3 
5  8  5 
6  9  2 
7  3  2 
8  2  7 
9  3  9 
10  8  4 
11  9  4 
12  5  7 
Columns A and B of the above spreadsheet on the right contain two arrays of data.
The probability associated with the Student's paired ttest with a onetailed distribution, for the two arrays of data can be calculated using the Excel Ttest function as follows :
This gives the result 0.449070689.
The probability associated with the Student's paired ttest with a twotailed distribution, for the same two arrays of data is calculated by :
This gives the result 0.898141379, which is double the probability associated with the onetailed test.
Further information and examples of the Excel Ttest function can be found on the Microsoft Office website.
If you get an error from the Excel TTEST Function, this is likely to be one of the following :
#N/A    Occurs if the two supplied arrays have different lengths 
#NUM!   
Occurs if either:

#VALUE!    Occurs if either the tails argument or the type argument is nonnumeric 