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Age as time varying covariate? |
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Dan posted on Wednesday, June 17, 2015 - 12:31 pm
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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 18-19; in Wave 2, age ranges from 19-20). 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 individually-varying 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 0-16). I'm trying to understand if keeping age as a TVC is OK, and further, trying to understand what's happening when I attempt individually-varying times of observation. Thanks for any direction you can provide! |
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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. |
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Dan posted on Tuesday, June 23, 2015 - 1:13 pm
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Thank you. I have a follow-up question about interpretation of time-varying 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 time-varying 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. |
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It's a standard regression with y1 as DV. Indirectly this changes the slope because the mean of y1 is affected. |
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Dan posted on Thursday, June 25, 2015 - 6:45 am
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Thank you! |
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