Hi Hywel,

I might be missing the point, but can't you just use the raw observed
proportions with random effects for lab and generation? A fixed effect for
control vs treatment will then give you the difference in hatching success.
If you start doing odd transformations to your dependent variable i'm not
sure you'll still be working with a binomial distribution.

On the other hand, a simple approach of taking means might be the way to go
depending on the structure of the data. Can you post a sample, or a dummy
set?


Andy.



andydolman@gmail.com


On 27 April 2010 15:26, Hywel Lloyd <hapdlloyd@googlemail.com> wrote:

> Hi, I've got stuck trying to model some data. Sorry for the trouble, but if
> anyone's got any help or suggestions I'd be really grateful.
>
> I'm trying to use mixed models to look at the rate that the hatching
> success
> of butterflies recovers after inbreeding over a number of generations. The
> data I have is the proportion of eggs that hatch for test and control
> populations so I would use a binomial distribution.
>
> But the main value I am interested in is the inbreeding depression, *s*,
> calculated as 1 - (test hatch rate/control mean hatch rate). This has two
> important roles - to give the relative decline in fitness and, by using the
> contemporaneous outbred hatching in each generation, it helps to control
> for
> other (lab) influences that may affect hatching from one generation to the
> next. The problem is though that occasionally the inbreds outperform the
> control, giving negative values for *s* which can't then be compared to the
> binomial distribution. Originally I simply used the test line mean hatch
> rates and assumed a normal distribution under the central limit theorem but
> this cut my number of data points from ~450 to 15 which seemed a bit
> wasteful.
>
> Does anyone have any suggestions for how I can proceed?
> I had considered simply changing the equation for *s* to 2 -
> (test/control),
> but I'm not sure if I could then convert my estimates for the fixed effect
> back into terms of the original inbreeding depression?
>
> Thanks for any help you can give me.
> Hywel
>
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