Mixed Models

Dear All, I am running a pop pk IV infusion data, the observed values are collected form obese patients and normal patients. I used WINONLIN to estimate the initial parameters. Patients who weigh more than 350 lbs are following 2CM and less than 350 are following 1CM. I confirmed the models based on AIC, CV% (of parameters), graphical representation. Vd increases with weight (1CM). So I used IF(WT<=350)THEN statements from the beginning ($MODEL, $ PK, $ DES, $THETA). I have the following error. AN ERROR WAS FOUND IN THE CONTROL STATEMENTS. AN ERROR WAS FOUND ON LINE 10 AT THE APPROXIMATE POSITION NOTED: IF(WT<=350)THEN X THE CHARACTERS IN ERROR ARE: IF Thank you very much in advance for your suggestions. Regards, Shankar Lanke Ph.D.