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Re: st: Graphing quadratic relationship


From   Ronan Conroy <rconroy@rcsi.ie>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Graphing quadratic relationship
Date   Fri, 18 Sep 2009 19:34:32 +0100

On 18 MFómh 2009, at 18:35, Stephanie L Kent wrote:

Dear users,

I would like to graph a relationship between a quadratic independent
variable and my dependent variable to see how y varies acccording to x and x^2. It's a logistic regression so my DV is 0-1 and I need to show how both the 0's and 1's vary according to x and x^2. Any advice on how to get
started is much appreciated!

I would be wary of quadratic terms, which tend to produce nonsense estimates at the extremes of the data, or to extrapolate to nonsense estimates beyond the observed range. Have you tried fractional polynomials?

As to graphing, I have two suggestions:

1. jitter your response variable to give a better impression of the density. If you have a lot of data, you can end up with two 'stripes' across the graph at y=0 and y=1 otherwise
2. Add a plot of predicted probabilities as a connected line




Ronan Conroy
=================================

rconroy@rcsi.ie
Royal College of Surgeons in Ireland
Epidemiology Department,
Beaux Lane House, Dublin 2, Ireland
+353 (0)1 402 2431
+353 (0)87 799 97 95
+353 (0)1 402 2764 (Fax - remember them?)
http://rcsi.academia.edu/RonanConroy

P    Before printing, think about the environment





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