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st: Compare Inverted U-shape curves

From   Alexander James <>
To   <>
Subject   st: Compare Inverted U-shape curves
Date   Wed, 9 May 2012 17:43:01 -0300

Dear Statalist members,

I am running a negative binomial model with random effects. My dependent variable is the count of new products released by a firm on year t. My main independent is a semi-continuous variable bounded between 0 and 1. In accordance with what I expected, I find a non-linear relationship (inverted U-shape) between my main independent and the dependent (the non-squared positive and the squared term negative). Moving a step further,
I interact the main IV (both squared and non-squared) with another continuous variable to see how it changes the inverted U-shape relationship. 

I was asked to plot the two curves (the interacted and the non-interacted) and compare how the curve changes. Would someone have a suggestion about how I should do it? Is there a user program in stata that allows me to do it?


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