R help - Jun 2014 - glm.fit: fitted probabilities numerically 0 or 1 occurred for a continuous variable?

I have gotten the this error before: "glm.fit: fitted probabilities
numerically 0 or 1 occurred"

and the problem was usually solved by combining one or more categories were
there were no observations.

I am now having this error show up for a variable that is continuous (not
categorical).

What could be the cause of this for a continuous variable??

Thanks, 
Nick



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On Jun 16, 2014, at 2:34 PM, Nwinters <nicholas.winters at mail.mcgill.ca>
wrote:
> I have gotten the this error before: "glm.fit: fitted probabilities
> numerically 0 or 1 occurred"
> 
> and the problem was usually solved by combining one or more categories were
> there were no observations.
> 
> I am now having this error show up for a variable that is continuous (not
> categorical).
> 
> What could be the cause of this for a continuous variable??
> 
> Thanks, 
> Nick

Presuming that this is logistic regression (family = binomial), the error is
suggestive of complete or near complete separation in the association between
your continuous IV and your binary response. This can occur if there is a
breakpoint within the range of your IV where the dichotomous event is present on
one side of the break and is absent on the other side of the break.

The resolution for the problem will depend upon first confirming the etiology of
it and then, within the context of subject matter expertise, making some
decisions on how to proceed.

If you Google "logistic regression separation", you will get some
resources that can be helpful.

Regards,

Marc Schwartz