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Inequality constraints in a growth model |
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Jon Heron posted on Tuesday, March 09, 2010 - 6:02 am
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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 |
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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. |
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Jon Heron posted on Tuesday, March 09, 2010 - 11:20 am
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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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