[R-sig-ME] Help with Linear Model

Phillip Alday Phillip.Alday at unisa.edu.au
Fri Nov 4 06:06:18 CET 2016

Hi Joseph,

Are you treating year as a continuous variable or a categorical? If you only have three years and one is outlier-isn, it may not make sense to treat them as a continuous variable (whether linear, quadratic or some other form).

Also, 2 sites is not enough levels for a random effect -- remember that RE are variance estimates and it doesn't really make sense to make a variance estimate of two levels. How many blocks do you have per site? Maybe Site:block is sufficient and if need be you can include Site as a nuisance parameter in the fixed effects?


> On 3 Nov 2016, at 05:07, Joseph Aidoo <jaidoo at ualberta.ca> wrote:
> My experiment was set up as a RCBD with
> 5 oat varieties
> 4 nitrogen rates
> at 2 sites
> over 3 years .
> I am investigating the effect of Variety and Nitrogen on Yield.
> In the second year of the experiment we had a drought so the data was
> awful. So i expect differences in Years
> I am using Variety, Nitrogen and Year as my fixed effects.
> and Sites as my random effect.
> I expect and interaction between Variety X Nitrogen.
> This is the model i came up with
> Yield ~ nitrogen*variety*Year + (Year|Site/block)
> My data doesnt seem normal because of the odd year with the drought.
> I have tried to normally transform the data but nothing seems to work.
> I need advice on what to do next? Is it ideal to use a glmm? What could the
> best model be?
> Thanks
> 	[[alternative HTML version deleted]]
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