Inequality constraints in a growth model PreviousNext
Mplus Discussion > Growth Modeling of Longitudinal Data >
 Jon Heron posted on Tuesday, March 09, 2010 - 12:02 pm
Sorry, this is my second question in as many days!

I'd like to fit a growth model to a repeated measure which cannot decrease - it's lifetime consumption of cigarettes.

I'd like to restrict the parameters such that the trajectories are monotone increasing - I'm thinking a piecewise linear with all positive gradients.

Is this possible by design, or do I just have to run the model + hope that I get what I'm after?

thanks, Jon
 Linda K. Muthen posted on Tuesday, March 09, 2010 - 4:46 pm
You may have problems with an outcome that cannot decrease. You may want to consider creating an outcome that represents the number of cigarettes smoked over a one year period.

You can place constraints using MODEL CONSTRAINT.
 Jon Heron posted on Tuesday, March 09, 2010 - 5:20 pm
Thanks Linda, I see I need to read the manual more carefully - inequalities are mentioned on P557.

It seems quite common in tobacco growth modelling (a discipline I am new to) to have a measure which lacks some of the usual metric properties - e.g. a 5-level ordinal variable where the first category of never-smoker can never be revisited once you've had at least one cigarette.
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