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06-05-2014 12:14 PM

Hello,

I wrote a formula in SAS IML. In that formula there are two type of parameters. One of them is from a existing file and the other one is created by SAS. When I run the program it works but the results are weird. I want to be sure that my code is correct. Could you check my code please. The formula which I work is attached.

data parms; | |||

INFILE "C:\mirt\parms.txt"; | |||

INPUT a1 a2 a3 d1; |

run;

proc iml;

call randseed(0);

use parms;

read all var {a1 a2 a3} into a;

read all var {d1} into d;

mean = {0,0,0}; *change these values to transfrom theta1 and theta2 to a different mean;

cov = {1 0.45 0.45,

0.45 1 0.45,

0.45 0.45 1}; *change the covariance to modify the correlation between dimensions; |

theta = RANDNORMAL(5000,mean,cov);

T=theta`;

z=j(5000,30);

p=j(5000,30);

u=j(5000,30);

do j=1 to 5000;

do i=1 to 30;

p[j,i]= exp((a[i,]*T[,j])+d[i,])/(1+(exp((a[i,]*T[,j])+d[i,]))); | ||||

u[j,i]=ranuni(0); |

if p[j,i]<u[j,i] then z[j,i]=0;

else if p[j,i]>=u[j,i]then z[j,i]=1;

end;

end;

create response from z;

append from z;

names1={theta1 theta2 theta3};

create theta from theta [colname=names1];

append from theta;

quit;

data keep2;

set response;

file "C:\mirt\mirt_data.txt";

put (col1-col30)(1.0);

run;

proc corr data=keep2 alpha;

var col1-col30;

ods output cronbachalpha=alpha;

run;

data alpha_need;

set alpha;

if _n_=1;

keep alpha;

file "c:\mirt\alpha\alpha.txt" mod;

put alpha;

run;

data theta;

set theta;

file "C:\mirt\theta.txt";

put theta1 theta2 theta3;

run;

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06-06-2014 11:43 AM

I don't know what "the results are weird" means. What are you seeing that is different than you expect?

Your loop appears to reconstruct the formula in the paper. I don't see anything obviously wrong, other than the code being slightly inefficient.

I suggest that you debug and check the code on a smaller example, like 5x3 instead of 5000x30. That will enable you to check the computations by hand to make sure they are correct.

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06-09-2014 07:22 AM

The program is much easier to understand (and more efficient) if you vectorize the computations instead of using two DO loops. In vector form, the main computation requires two calls:

p = logistic( theta*A` + d` );

call randgen(z,"Bernoulli", p);

The computation of P is equivalent to the longer computation that you have:

do j=1 to 5000;

do i=1 to 30;

p[j,i]= exp((a[i,]*T[,j])+d[i,])/(1+(exp((a[i,]*T[,j])+d[i,])));

end;

end;

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06-09-2014 01:27 PM

Dr. Wicklin,

Thank you SO much for your answer. I really appreciate it. I will work on both method. By the way "weird results" means different results from literature. Now, I'm pretty sure these results are because of my design.