

Age as time varying covariate? 

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Dan posted on Wednesday, June 17, 2015  12:31 pm



Hi, Is it possible & appropriate to include age as a time varying covariate in LGCA? I have a dataset with 4 ~"equally" spaced waves, although there is about 1 year range in ages within a given wave (e.g., in Wave 1, age ranges from 1819; in Wave 2, age ranges from 1920). 1) Modeling a linear LGCA: When I include AGE as a time varying covariate in the linear model, the fit is very good as suggested by CFI, TLI, SRMR, RMSEA, and lower BIC. [As a sidenote, AGE is not a significant covariate at any wave in this model] However, when I remove AGE from the time varying covariate list, the model fit becomes abysmal as indicated by the various indicators. 2) So, I tried LGCA with individuallyvarying times of observation, using AGE as a the TSCORES. However, I get a giant residual variance for ICEPT (200+, when outcome Y is only scaled 016). I'm trying to understand if keeping age as a TVC is OK, and further, trying to understand what's happening when I attempt individuallyvarying times of observation. Thanks for any direction you can provide! 


I would use TSCORES, but make sure you have age transformed to have a zero value at the start, and perhaps divide it by 10 to get an easier scale for the iterations. 

Dan posted on Tuesday, June 23, 2015  1:13 pm



Thank you. I have a followup question about interpretation of timevarying covariates. A current growth curve model I'm working with is similar to Example 6.10:  i s  y0@0 y1@1 y2@2; i s ON x1 x2 x3; y0 ON a0 b0 c0; y1 ON a1 b1 c1; y2 ON a2 b2 c2;  What would be the proper interpretation of a significant timevarying covariate (e.g., a1) in the y1 regression? Is it just a standard regression where a1 influences the y1? Or is "a1" exerting an effect on the slope? [everything is continuous] Thanks, and sorry if I missed this elsewhere on the board. 


It's a standard regression with y1 as DV. Indirectly this changes the slope because the mean of y1 is affected. 

Dan posted on Thursday, June 25, 2015  6:45 am



Thank you! 

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