[R] logistic regression

[Bert Gunter]

Get help. You do not understand glm's. What do you think the fitted
values are? -- Hint: they are *not* an estimate of the number of
living fruit flies.

-- Bert

On Tue, Apr 3, 2012 at 6:25 PM, Melrose2012
<melissa.patrician at stonybrook.edu> wrote:
> I am trying to plot the logistic regression of a dataset (# of living flies
> vs days the flies are alive) and then fit a best-fit line to this data.
>
> Here is my code:
> plot(fflies$living~fflies$day,xlab="Number of Days",ylab="Number of Fruit
> Flies",main="Number of Living Fruit Flies vs Day",pch=16)
> alive <- (fflies$living)
> dead <- (fflies$living[1]-alive)
> glm.fit <- glm(cbind(alive,dead)~fflies$day,family="binomial")
> summary(glm.fit)
> lines(sort(fflies$day),fitted(glm.fit) [order (fflies$day)],col =
> "red",lwd=2)
> lines(glm.fit,col = "red",lwd=2)
>
> My problem is that, while I am pretty sure that I did the 'glm' command
> correctly, when I try to plot this as a best-fit line on my data, it does
> not fit (clearly on a different scale somehow).
>
> Can anyone enlighten me about what I am doing wrong?  Should I be scaling
> one of these somehow?  I've tried various types of scaling but nothing plots
> the line on top of the plot (no matter which I scale).
>
> Thanks so much for whatever help you can give me!
>
> Cheers,
> Melissa
>
> --
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>
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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