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Mplus Discussion > Structural Equation Modeling >
 Stefanie Hinkle posted on Tuesday, August 16, 2011 - 2:40 pm
Hi. I running a path model to examine the relationship between x and y while including covariates and mediators. I would like to account for a non-linear relationship between x and y, as indicated when I examine the relationship using ordinary regression. Do you have any recommendations on how to best account for a non-linear relationship? I have tried to include X^2 in the model, having it modeled just as X is and allowing the two to covary. However the model is non-positive definite when I do this.

I have also considered including an additional term for piecewise regression. Would it follow the normal X term and do they covary?

Any advice that you could provide would be very helpful!

Thanks so much!
 Bengt O. Muthen posted on Wednesday, August 17, 2011 - 11:32 am
Instead of X and X^2 try using Z and Z2,

Z = X-X_bar
Z2 = Z*Z

where X_bar is the sample mean of X. If that gives an error, send to support.
 Susanne Meiser posted on Monday, March 07, 2016 - 4:08 am
Dear Linda/Bengt

I would like to test a threshold model (in which the relationship between X and Y gets significantly stronger above a certain value of X) using piecewise/segmented regression. Also I would prefer the threshold value to be estimated from the data instead of fixing one a priori. Can this be done in Mplus?

Thank you so much for any help or advice!
 Tihomir Asparouhov posted on Tuesday, March 08, 2016 - 9:53 am
You can try mixture modeling as in User's guide example 7.1. This would be our recommendation for you.

You can also do piece wise segmentation - where you would manually select the threshold value and choose the one with the best likelihood (from a grid of threshold values). If you decide to do that you can still compare the model to the mixture model using BIC.
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