[R] Finding LD50 from an interaction Generalised Linear model

[Gduncan]

Many thanks indeed Bill. I understand now what you're getting at with the
a/b-1!

Many thanks again.
Graeme Duncan


Bill.Venables wrote:
> 
> The trick is to fit the model in a form which has the two separate
> intercepts and the two separate slopes as the parameters.
> 
> You do have to realise that a*b, a*b-1, a/b, a/b-1, ... all specify the
> same model, they just use different parameters for the job.  (Yes,
> really!)
> 
>> dat
>    ldose sex numdead
> 1      0   M       0
> 2      1   M       3
> 3      2   M       9
> 4      3   M      16
> 5      4   M      18
> 6      5   M      20
> 7      0   F       0
> 8      1   F       2
> 9      2   F       6
> 10     3   F      10
> 11     4   F      11
> 12     5   F      14
>> dat <- transform(dat, Tot = 20)
>> fm <- glm(numdead/20 ~ sex/ldose-1, binomial, dat, weights = Tot)
>> coef(fm)
>       sexF       sexM sexF:ldose sexM:ldose 
> -2.7634338 -3.4853625  0.7793144  1.5877754 
>> dose.p(fm, c(1,3))           ## females
>              Dose        SE
> p = 0.5: 3.545981 0.3025148
>> dose.p(fm, c(2,4))		 ## males
>              Dose        SE
> p = 0.5: 2.195123 0.1790317
>>  
> 
> In fact if you look at the book from which dose.p comes, you will find
> an example not unlike this one done this way.  At least I think that's
> what I, er, read.
> 
> W.
> 
> Bill Venables
> CSIRO Laboratories
> PO Box 120, Cleveland, 4163
> AUSTRALIA
> Office Phone (email preferred): +61 7 3826 7251
> Fax (if absolutely necessary):  +61 7 3826 7304
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> mailto:Bill.Venables at csiro.au
> http://www.cmis.csiro.au/bill.venables/ 
> 
> 

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