chi2calc.m

function [p] = chi2calc(x,df)
%CHI2CALC - Returns the probability of a chi-square value
%
%   function with V degrees of freedom at the values in X.
%   The chi-square density function with DF degrees of freedom,
%   is the same as a gamma density function with parameters V/2 and 2.
%
%   The size of P is the common size of X and V. A scalar input
%   functions as a constant matrix of the same size as the other input.
%
%   This program calculates the chi-square significance values for given
%   degrees of freedom and the tail probability (type I error rate) for
%   given observed chi-square statistic and degree of freedom.

% Molecular Biology and Evolution Toolbox (MBEToolbox)
% Author: James Cai
% Email: jcai@tamu.edu
% Website: http://bioinformatics.org/mbetoolbox/
% 
% $LastChangedDate: 2013-01-05 12:04:29 -0600 (Sat, 05 Jan 2013) $
% $LastChangedRevision: 327 $
% $LastChangedBy: jcai $


p=1-chi2cdf(x,df);

% find critical value
% critical = chi2inv(1-alpha, df);
% chi2inv(1-0.05,1)=3.8415