Thank you for the reply! When I run your codes I met another problem. I can get parameter estimates for intercept, disease, year, and disease*year. But when I add some other covariates into the model, i found that the estimates for intercept and disease changed but the estimate for disease*year remained the same. I am not quite sure if this is correct? (I think the estimate for disease*year should change and the estimates for intercept and disease should remain the same).
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