[R] analysing a three level reponse

[Robert A LaBudde]

The best method would probably be proportional odds regression using 
polyr() in 'MASS'.

At least it's a starting point.


At 09:19 AM 6/22/2011, Matt Ellis \(Research\) wrote:
>Hello,
>I am struggling to figure out how to analyse a dataset I have inherited
>(please note this was conducted some time ago, so the data is as it is,
>and I know it isn't perfect!).
>
>A brief description of the experiment follows:
>Pots of grass were grown in 1l pots of standad potting medium for 1
>month with a regular light and watering regime. At this point they were
>randomly given 1l of one of 4 different pesticides at one of 4 different
>concentrations (100%, 75%, 50% or 25% in water). There were 20 pots of
>grass for each pesticide/concentration giving 320 pots. There were no
>control (untreated) pots. The response was measured after 1 week and
>recorded as either:
>B1 - grass dead
>B2 - grass affected but not dead
>B3 - no visible effect
>
>I could analyse this as lethal effect vs non-lethal effect (B1 vs B2+B3)
>or some effect vs no effect (B1+B2 vs B3) binomial model, but I can't
>see how to do it with three levels.
>
>Any pointing in the right direction greatly appreciated!
>Thanks
>Matt
>
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================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral at lcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
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