[R] nonmonotonic glm?

Ben Bolker bbolker at gmail.com
Sun Jan 11 23:00:04 CET 2015

Stanislav Aggerwal <stan.aggerwal <at> gmail.com> writes:

> I have the following problem.
> DV is binomial p
> IV is quantitative variable that goes from negative to positive values.
> The data look like this (need nonproportional font to view):

  [snip to make gmane happy]

> If these data were symmetrical about zero, 
> I could use abs(IV) and do glm(p
> ~ absIV).
> I suppose I could fit two glms, one to positive and one to negative IV
> values. Seems a rather ugly approach.


  What's wrong with a GLM with quadratic terms in the predictor variable?

This is perfectly respectable, well-defined, and easy to implement:


or   y~x+I(x^2)  or y~poly(x,2,raw=TRUE)

> (To complicate things further, this is within-subjects design)

glmer, glmmPQL, glmmML, etc. should all support this just fine.

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