Hi,
I have a question about a comparison of two groups of data, which are small count. The following are the code. However, when I fit the model, the "contrast" p-value cannot be calculated. And SAS always give me the warning:
WARNING: The negative of the Hessian is not positive definite. The convergence is questionable.
WARNING: The procedure is continuing but the validity of the model fit is questionable.
WARNING: The specified model did not converge.
WARNING: Negative of Hessian not positive definite.
Is there any problem with my code? The following is the data and the code:
data test;
input diet total;
cards;
1 1
1 0
1 1
1 1
1 2
1 0
1 0
1 4
1 1
1 1
1 1
1 3
1 1
1 3
1 0
1 3
1 1
1 3
1 1
2 3
2 2
2 2
2 2
2 4
2 1
2 3
2 4
2 1
2 3
2 0
2 5
2 2
2 6
2 2
2 1
2 2
2 2
2 2
2 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
4 4
4 5
4 4
4 3
4 4
4 7
4 4
4 3
4 2
4 4
4 1
4 2
4 2
4 3
4 4
4 0
4 2
4 5
;
proc genmod data=test;
class diet;
model total=diet / dist=poisson link=log;
contrast 'group, 1 vs. 2' diet 1 -1 0 0;
contrast 'group, 1 vs. 3' diet 1 0 -1 0;
contrast 'group, 1 vs. 4' diet 1 0 0 -1;
run;
