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I would like to estimate a Multiple Cohort Growth Model similar to the example in ch 6.18 of the user guide. Instead of cohorts based on birth year, my three cohorts are grouped according to grade at baseline – 3rd, 6th, and 9th grade. So ages within each cohort do range at each assessment (each cohort was assessed at three timepoints – 18 months apart). Can I use the syntax from 6.18 asis or are there any adjustments I need to make to how my data is structured or to the syntax? 


Below is a draft of my syntax, thanks! Note: APD is the outcome whose trajectory I want to model. GROUPING = g (1 = 3RD 2 = 6TH 3 = 9TH ); MODEL: i s APD1@0 APD2@.15 APD3@.3; [i] (1); [s] (2); i (3); s (4); i WITH s (5); APD1 ON age1; APD2 ON age2; APD3 ON age3 (g6); APD1APD2; APD3 (v6); MODEL SIXTH: i s y1@.3 y2@.45 y3@.6; APD1 ON age1 (g6); APD2 ON age2; APD3 ON age3 (g9); APD1 (v6); APD2; APD3 (v9); MODEL NINTH: i s y1@.6 y2@.75 y3@.9; APD1 ON age1 (g9); APD2 ON age2; APD3 ON age3; APD1 (v9); APD2APD3; 


I also want to clarify that my primary goal is to model the trajectory of APD over 7 timepoints from grade 3rd to 12th (ages 8 to 18). 


You don't need to include age as a predictor unless such small differences within grade are substantively important. And isn't it the case that age2 = age1 + 1.5 etc for age3? If so, these age variables are correlated 1 which is a problem. You need only the first one. Using 6.18 is fine  your input looks ok. Note that we request that postings be limited to one window only. For longer mesages, send to Support. 


Thank you, this is helpful. You are correct, age2 = age1 + 1.5 and so on. Would you then suggest dropping the time varying covariates (ages) from the model? Or just using one age variable (e.g., Age1) since they are all correlated as you mention? If the latter, would I just include the one regression (APD1 ON age1;) in my models, and drop the other two regressions? I am surprised that this was not an issue in Ex 6.18 as I thought the ages there would be correlated as well. 


You can use Age1. Ex 6.18 has timevarying covariates labeled "a" but they are not intended to reflect age. 


Ok, thank you. What do the time varying covariates "a" in Ex 6.18 represent? Would it just be another outcome variable (e.g., if my main outcome is depression, a tvc of interest may be anxiety) ? 


In an education application you might have y = math achievement in a certain grade a = math course taking in a certain grade 

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