The Excel T.INV.2T function calculates the inverse of the twotailed Student's T Distribution, which is a continuous probability distribution that is frequently used for testing hypotheses on small sample data sets.
The T.INV.2T function is new to Excel 2010. However, this function simply replaces the TINV function, that is available in earlier versions of Excel.
The format of the function is :
Where the function arguments are:
probability    The probability (between 0 and 1) for which you want to evaluate the inverse of the twotailed Student's T Distribution. 
degrees_freedom    The number of degrees of freedom (must be ≥ 1) 
Excel uses an iterative method to calculate the T.Inv.2t function, and seeks to find a result, x, such that T.DIST.2T( x, degrees_freedom ) = probability.
Note that the Excel T.Inv.2t Function calculates the inverse of the twotailed Student's T Distribution.  if you want to calculate the inverse of the onetailed tdistribution you can simply double the probability.
ie. The onetailed inverse Student's T Distribution = T.INV.2T( 2*probability, degrees_freedom ).
The above chart on the right shows the twotailed Inverse Student's T Distribution with 10 degrees of freedom.
If you want to calculate the value of this function for a probability of 0.25, this can be done using the Excel T.Inv.2t function, as follows:
This gives the result 1.221255395.
If you want to calculate the value of the onetailed inverse Student's TDistribution for a Probability of 0.25, this can be done by doubling the probability, as follows :
which gives the result 0.699812061.
Further information and examples of the Excel T.Inv.2t function can be found on the Microsoft Office website.
If you get an error from your Excel T.Inv.2t function this is likely to be one of the following :
#NUM!   
Occurs if either:

#VALUE!    Occurs if either of the supplied arguments are nonnumeric 