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Dear Drs Muthen, I am analyzing data from a large clinical trial with three arms: one Placebo (pl) and two treatments (t1; t2). I am applying the approach explained in Muthen & Curran (1997). In addition to "treatment effects" (pl vs tr1 and pl vs tr2) I am interested in examining the effect of a continuous covariate (X) on the <continuous> outcome within each grp (pl, tr1, & tr2). The model without the covariate does not converge. The ssize in each grp varies from 200 to 290. (below is my script.) My questions are: 1) Does the approach explained in Muthen & Curran can be extended to two treatment arms vs placebo? If so, would you please check my syntax? 2) How would you add the effect of the covariate into this modeling approach? Many thanks for your help!! 


here is the code: i s  v0@0 v18@1.5 v36@3 v48@4; i tr1  v0@0 v18@1.5 v36@3 v48@4; i tr2  v0@0 v18@1.5 v36@3 v48@4; [v0v48] (1); [i@0]; i (2); s (3); i WITH s (4); [s] (5); tr1@0; tr2@0; q WITH i@0 s@0; tr1@0; tr2@; v1y36 WITH v2v48; tr1 ON i; tr2 on i; tr2 with tr1@0; MODEL control: tr1 ON i@0; tr2 on i@0; [tr1@0]; [tr2@0]; 


1) Yes. You have to use 3 groups instead of 2. 2) The covariate effect could be constant over the groups, so specified as i s on x in the overall part of the model. Your input mentions "q" but you don't show quadratic growth. 


Dear Dr Muthen, Thanks for your response. The q is a typo, I don't have quadratic growth. I ran the following model with and without the covariate "PL727" and I get a "nonconvergent  possibly nonidentified model." I tried fixing a negative residual variance to a low number and it didn't work. Is there something obviously wrong with my code? i s  f120@0 f1218@1.5 f1236@3 f1248@4; i e2  f120@0 f1218@1.5 f1236@3 f1248@4; i cee  f120@0 f1218@1.5 f1236@3 f1248@4; i s on PL727; [f120f1248] (1); [i@0]; i(2); s(3); i WITH s (4); [s] (5); e2@0; cee@0; f120f1236 WITH f1218f1248; e2 on i; cee on i; cee with e2@0; MODEL Placebo: e2 ON i@0; cee on i@0; e2 with cee@0; [e2@0]; [cee@0]; 


You should have three groups not two if you have two treatements and one placebo. Please send the output and your license number to support@statmodel.com. 

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