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Quadratic association

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Quadratic association

Hi all,


I am looking for the best way to depict a concave, quadratic, rather than linear association. I'm using logistic regression to measure the association between affect and military advancement (yes/no).


The primary predictor centered on the mean squared was significant, and it is graphically clear that Y increases as X increases only to a point, before leveling off and decreasing. I was able to determine at which level of X, Y reaches its maximum by adding the mean to the linear term (centered on the mean), divided by two times the quadratic term (mean + (b1/2b2). 


Are results from logistic regression uninterpretable if there is a curvilinear/quadratic association between X and Y?


What would be the best way in SAS to determine the slope at different levels of X for a binary outcome? I would like to compare how the slope differs at varying levels of X (low vs. medium vs. high), and specifically how it decreases at higher levels of X. 






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