[R-sig-ME] Time as both fixed and random term

[shi_peijian]

Dear all,


Could I additonally ask a question?
    fit1 <- Biomass ~ Treatment + Time + (1|Plot), where 'Time' is a continuous covariate
    fit2 <- Biomass ~ Treatment  + (1|Time/Plot), where 'Time' is a factor variable


    fit3 <- Biomass ~ Treatment + (1|Time), where 'Time' is also a factor variable


If AIC(fit3) is smaller than AIC(fit1) and AIC(fit2), can we choose fit3 rather than fit1?


Thanks a lot!


Best regards,


Joe



--


Peijian (Joe)  Shi, Ph.D.

Research interests: forest ecology; theoretical ecology; thermal biology

Member of China Ornithological Society from 2005 up to the present

Bamboo Research Institute, Nanjing Forestry University, P.R. China

159 Longpan Road, Nanjing City, Jiangsu Province 210037

Office:  60817  Biotechnology Building

Tel:  +86 25 85427231 

E-mail addresses:  peijianshi at gmail.com  

                               shi_peijian at 163.com      




At 2015-11-25 06:06:51, "Lionel" <hughes.dupond at gmx.de> wrote:
>Dear List,
>
>In my work we usually deals with measures sampled repeatedly on 
>experimental units over several time points and with specific 
>treatments. For example we sampled plant biomass on 100 experimental 
>plots at 5 different time point (say season or year). Some people argue 
>that in this context we should model time as both a fixed effect term 
>(as continuous variable) and random effect term in order to compute the 
>correct numbers of degrees of freedom to test our treatment effects 
>(usually considered as a continuous variables).
>
>This is how such a model would look like:
>
>Biomass ~ Treatment + Time + (1|Plot) + (1|Time)
>
>In my experience having the same term has both fixed and random results 
>in very low estimated standard deviation for the random term, which 
>makes me skeptical about this approach. But having very little knowledge 
>about how to correctly estimate the numbers of degrees of freedom I 
>would like to ask you:
>
>(i) if such a model makes sense,
>(ii) if the argument "we need to have time as both fixed and random term 
>to get the correct number of degrees of freedom" is valid
>(iii) if such an alternative model: "Biomass ~ Treatment + Time + 
>(1|Plot)" would be more appropriate.
>
>Thanks for your input,
>Lionel
>
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