Chi squared distribution

[shakuntalaParticipant]

% chiprob.m
% @shortDesc: Distribucion CHI^2 . Enviado por M. Urrutia MD, PhD.
% @fullDesc: Usa un algoritmo de expansión en serie para el cálculo de probabilidades exactas de la distribucion CHI^2 en funcion de V grados de libertad P(X>=x).Ref. Abramowit and Stegun. Handbook of Mathematical Functions. Dover Publications Inc. NY
% z=chiprob(Chivalue,GL)
function [xp] =chiprob(x,y)

n1 = power(10,-100);
n2 = power(10,100);
denom = 1;
chisqr= x;
df = y;
dx = y;
vx =y;
parity=0.0;
while df >=2
denom = denom * df;
df = df -2;
end
numer=power(chisqr,(floor((vx+1)/2))) *exp(-chisqr / 2) / denom;
if round(vx/2) != (vx/2 )
parity = sqrt(2 / chisqr / pi);
else
parity = 1.0;
end
s = 1.0;
t= 1.0;
while t > 0.0000001
dx = dx + 2;
t = t * chisqr/ dx;
s = s + t;
if ((s<= n1) | (s >= n2)) | ((t <= n1) | (t>= n2 ))
end
end
%limite superior
xp = 1-abs((parity*numer*s));
end

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