This article describes the formula syntax and usage of the TTEST function in Microsoft Excel.
Returns the probability associated with a Student's tTest. Use TTEST to determine whether two samples are likely to have come from the same two underlying populations that have the same mean.
Important: This function has been replaced with one or more new functions that may provide improved accuracy and whose names better reflect their usage. Although this function is still available for backward compatibility, you should consider using the new functions from now on, because this function may not be available in future versions of Excel.
For more information about the new function, see T.TEST function.
Syntax
TTEST(array1,array2,tails,type)
The TTEST function syntax has the following arguments:

Array1 Required. The first data set.

Array2 Required. The second data set.

Tails Required. Specifies the number of distribution tails. If tails = 1, TTEST uses the onetailed distribution. If tails = 2, TTEST uses the twotailed distribution.

Type Required. The kind of tTest to perform.
If type equals 
This test is performed 
1 
Paired 
2 
Twosample equal variance (homoscedastic) 
3 
Twosample unequal variance (heteroscedastic) 
Remarks

If array1 and array2 have a different number of data points, and type = 1 (paired), TTEST returns the #N/A error value.

The tails and type arguments are truncated to integers.

If tails or type is nonnumeric, TTEST returns the #VALUE! error value.

If tails is any value other than 1 or 2, TTEST returns the #NUM! error value.

TTEST uses the data in array1 and array2 to compute a nonnegative tstatistic. If tails=1, TTEST returns the probability of a higher value of the tstatistic under the assumption that array1 and array2 are samples from populations with the same mean. The value returned by TTEST when tails=2 is double that returned when tails=1 and corresponds to the probability of a higher absolute value of the tstatistic under the “same population means” assumption.
Example
Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.
Data 1 
Data 2 

3 
6 

4 
19 

5 
3 

8 
2 

9 
14 

1 
4 

2 
5 

4 
17 

5 
1 

Formula 
Description (Result) 
Result 
=TTEST(A2:A10,B2:B10,2,1) 
Probability associated with a Student's paired tTest, with a twotailed distribution. Uses values in A2:A10 and B2:B10. 
0.19602 