Solution 11717: Calculating the Inverse Student-T (invT) Probability on the TI-83 Plus Family of Graphing Calculators.
How can I calculate the inverse student-T probability on the TI-83 Plus family of graphing calculators?
The Inverse Student-T is usually used for determining probabilities of population means from sampled distributions when the standard deviation of the population is unknown. Specifically, t = (x-u)*sqrt(n)/s with n-1 degrees of freedom and where x is the sampled mean, u is the population mean, s is the sample standard deviation, n is the sample size, and the population is Gaussian.
The command for this function is under the DISTR menu. Please refer to the instructions provided below given the below data.
Area = 0.68759
DF = 18
1) Press [2nd][DISTR]
2) Press  to select 4:InvT(
3) Press  [.]      [,]   [ ) ]
4) Press [ENTER] to display the answer of 0.4975567034
Please see the TI-83 Plus Family guidebooks for additional information.