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
=================================
[email protected]
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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