Modeling with Time-Varying Covariates PreviousNext
Mplus Discussion > Growth Modeling of Longitudinal Data >
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 Bengt O. Muthen posted on Tuesday, August 05, 2008 - 3:04 pm
This exercise considers growth in mathematics achievement over grades 7, 8, 9, and 10 in U.S. public schools for a sample of 3,102 students. The data are from the Longitudinal Study of American Youth (LSAY). The data structure is multilevel with students clustered within schools, but for the purpose of this assignment this complication can be ignored.

The research question is how the time-varying covariate of math course taking influences the math achievement growth. The variables in the attached LSAY data set are shown in the attached Mplus input for a BASIC analysis. Here, the math course taking variable is labeled mthcrs7-mthcrs10 and is recorded as follows:

mthcrs7-mthcrs12 = Highest math course taken during each grade
! (0 = no course, 1 = low, basic, 2 = average, 3 = high,
! 4 = pre-algebra, 5 = algebra I, 6 = geometry,
! 7 = algebra II, 8 = pre-calc, 9 = calculus)

For simplicity, this variable may be treated as continuous.

The exercise consists of using Mplus to find a good time-varying covariate growth model by exploring conventional approaches with fixed and random coefficients, as well as innovative alternatives. Individuals with little Mplus growth modeling experience may consult chapter 6 of the Mplus Version 5 Userís Guide (see www.statmodel.com).

Although it is not possible for the Mplus team to look at individual solutions, the Mplus course at Johns Hopkins University on Thursday August 21, 2008 will present several solutions for this exercise.


Bengt Muthen
 Scott Smith posted on Friday, October 18, 2013 - 1:13 pm
Hello,

I am trying to model a growth curve model with a count dependent variable. I have four waves of data but I am using an accelerated cohort design. I already have my data wide. I want to use both time-varying and time-invariant covariates. I used example 6.10 as a guide. If I only have the time-invariant variable in the model (i.e. gender) the model will run. When I start to add the time-varying covariates (i.e. drug use at each time point) I get the following message:


*** ERROR
There is at least one count variable that has only one unique value.
Please check your data and format statement.
*** ERROR
One or more variables in the data set have no non-missing values.
Check your data and format statement.

This is my syntax


model:
i s| dv16@0 dv17@1 dv18@2 dv19@3 dv20@4 dv21@5 dv22@6 dv23@7;
i s ON female;
dv16 ON drugs16 ;
dv17 ON drugs17 ;
dv18 ON drugs18 ;
dv19 ON drugs19 ;
dv20 ON drugs20 ;
dv21 ON drugs21 ;
dv22 ON drugs22 ;
dv23 ON drugs23 ;

Thank you for any advice can give me.
 Bengt O. Muthen posted on Friday, October 18, 2013 - 8:52 pm
Perhaps you have a lot of missing data on drugs16-drugs23. If this is the cause of your problem and the missingness causes missing also on the corresponding dv's, we can give you a solution.
 Scott Smith posted on Saturday, October 19, 2013 - 9:08 am
I believe you are right because I am using an accelerated cohort design. What would you suggest I do to help with the missing data? I really appreciate any advice.
 Bengt O. Muthen posted on Saturday, October 19, 2013 - 3:33 pm
Don't use a missing data flag for the time-varying covariates; use some other number. If the tvs is missing at time t and the outcome at the corresponding time is missing, such timepoints still won't contribute to the likelihood computations. By not using a missing data flag, you avoid deleting subjects who have missing on any tvcs.
 Scott Smith posted on Saturday, October 19, 2013 - 5:12 pm
That makes sense. I will give it a try. Thank you for your timely feedback.
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