In MATLAB: find two tailed P value of t-distribution (Student's t-distribution)

Problem:
-----------
Given the t-ratio and the degrees of freedom(dof), find two tailed P value of t-distribution (Student's t-distribution)

Solution:
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Using the econometrics toolbox here is an example where t ratio = 2.2000 and degree of freedom= 5 ; The computed two tailed P Value is 0.0791

function y = tdis_prb(x,n)
% PURPOSE: calculates t-probabilities for elements in x-vector
%---------------------------------------------------
% USAGE: y = tdis_prb(x,n)
% where: x = vector containing computed t-values
%        n = degrees of freedom parameter
%---------------------------------------------------
% RETURNS:
%        y = a vector of marginal probability levels
% --------------------------------------------------
% SEE ALSO: fdis_prb(), chis_prb
%---------------------------------------------------

% written by:
% James P. LeSage, Dept of Economics
% University of Toledo
% 2801 W. Bancroft St,
% Toledo, OH 43606
% jpl@jpl.econ.utoledo.edu


if nargin ~= 2; error('Wrong # of arguments to tdis_prb'); end;
if n <=0; error('dof is negative or zero in tdis_prb'); end;

x2 = n./(n+x.^2);
one = find(x2 >= 1);
if length(one) > 0
    x2(one,1) = 1-1e-12;
end;
zip = find(x2 <= 0);
if length(zip) > 0
    x2(zip,1) = 1e-12;
end;

tmp = 1.0 - 0.5*betainc(x2,0.5*n,0.5);
y = 2*(1-tmp);
x
n
end

x =

    2.2000


n =

     5


ans =

    0.0791

Published with MATLAB® 7.13

Exercise:
-----------
Using this function (tdis_prb) plot results for [-4:0.05:4] for 10 degrees of freedom.