# [R] Inverse Student t-value

Duncan Murdoch murdoch.duncan at gmail.com
Tue Sep 30 20:20:39 CEST 2014

```On 30/09/2014 2:11 PM, Andre wrote:
> Hi Duncan,
>
> No, that's correct. Actually, I have data set below;

Then it seems Excel is worse than I would have expected.  I confirmed
R's value in two other pieces of software,
OpenOffice and some software I wrote a long time ago based on an
algorithm published in 1977 in Applied Statistics.  (They are probably
all using the same algorithm.  I wonder what Excel is doing?)

> N= 1223
> alpha= 0.05
>
> Then
> probability= 0.05/1223=0.0000408831
> degree of freedom= 1223-2= 1221
>
> So, TINV(0.0000408831,1221) returns 4.0891672
>
>
> Could you show me more detail a manual equation. I really appreciate
> it if you may give more detail.

I already gave you the expression:  abs(qt(0.0000408831/2, df=1221)).
For more detail, I suppose you could look at the help page for the qt
function, using help("qt").

Duncan Murdoch

>
> Cheers!
>
>
> On Wed, Oct 1, 2014 at 1:01 AM, Duncan Murdoch
> <murdoch.duncan at gmail.com <mailto:murdoch.duncan at gmail.com>> wrote:
>
>     On 30/09/2014 1:31 PM, Andre wrote:
>
>
>         I am trying to use calculation for two-tailed inverse of the
>         student`s
>         t-distribution function presented by Excel functions like
>         =TINV(probability, deg_freedom).
>
>         For instance: The Excel function =TINV(0.0000408831,1221) =
>         returns
>           4.0891672.
>
>         Would you like to show me a manual calculation for this?
>
>
>
>     That number looks pretty far off the true value.  Have you got a