How can I fit a linear mixed-effects model with only quadratic/second-order terms?
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I'm trying to fit a linear mixed-effects model with fitlme, e.g.: quadmdl = fitlme(Data,'DV ~ X1^2 + (1 + X1^2 | Subject)');
This models fixed effects for both X1 and X1^2, but I only want to model X1^2. In other words, I want my model to have the quadratic (second-order) term without the linear (first-order) one: DV ~ X1^2 + (1 + X1^2 | Subject) as opposed to DV ~ X1 + X1^2 + (1 + X1 + X1^2 | Subject).
Is there a way to specify this in the model?
Peng Li on 6 Aug 2020
This is an old post now. Just for future readers, you can explicitly get rid of any items you don't want by minus it. For the questions above, the formula can be DV ~ X1^2 - X1 + ...