[R] logit GLM without intercept
Prof Brian Ripley
ripley at stats.ox.ac.uk
Tue Apr 22 17:54:02 CEST 2008
On Tue, 22 Apr 2008, gorgasal at gmx.de wrote:
> Hi Robert,
> you can exclude the intercept by including "-1" in the formula:
> value <- as.numeric(runif(20)<.4)
> ppm <- rnorm(20)
Yes, but that is the same as 0 + ppm, which I tend to think is more
In so far as I understand the original question (what is 'y'?), either
0+ppm or ppm-1 will predict probability p = 0.5 at ppm = 0.
>> Dear Statisticians,
>> I would like to analyse my data with a GLM with binomial error distribution
>> and logit link function. The point is that I want a model fitted without
>> intercept, i.e. the fitted curve should start at y=0.5 for x=0.
>> I tried it with the following code:
>> glm(value~0+ppm, binomial)
>> Does this code yield the correct model or is there another possibility? I?d
>> appreciate it very much if you could help me out with this. I attached some
>> example data.
>> Thanks & all the best
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Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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