For information of trivariate distribution to bivariate distribution see,
http://www.sci.wsu.edu/math/faculty/genz/papers/bvnn/node3.html
proc iml;
start fun(t) global (b2, b3, rho12, rho13, rho23);
x1=(b2 - rho12#t)/sqrt((1-rho12#rho12));
x2=(b3 - rho13#t)/sqrt((1-rho13#rho13));
rho=(rho23 - rho12#rho13)/sqrt((1-rho12#rho12)#(1-rho13#rho13));
f=probbnrm(x1,x2,rho);
v=exp(-0.5 # (t # t) ) # f;
return(v);
finish;
b1=0;
b2=0;
b3=0;
rho12=0.99992;
rho13=0.64627;
rho23=0.63975;
*print b1 b2 b3 rho12 rho13 rho23;
interval= .M || b1;
call quad(z,"fun",interval);
prob2=z/(sqrt(2#constant('pi')));
prob_theory=0.125 +(0.25/constant('pi'))*(arsin(rho12) + arsin(rho13) + arsin(rho23) );
print prob2[format=E21.14] prob_theory[format=E21.14];
quit;



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