[R-sig-ME] Rules of thumb for model complexity with small sample size in lme()

[Ben Bolker]

  As I interpret the description, there is actually replication at the
transect level; ~ 2 samples per transect (30 transects, total N=66). In
principle one could even ask if there are variations in the effect of N
and P across transects (this is essentially a very unbalanced
randomized-block design), but I agree that would be unrealistically
optimistic.

  Otherwise I agree with Thierry.

On 18-04-04 11:10 AM, Thierry Onkelinx wrote:
> Dear John,
> 
> Since you don't have replication at the transect level, you should
> omit that from the random effects structure.
> 
> I tend to strive for at least 10 observations per parameter. More is
> better of course. Assuming that Nitrogen and Phosphorus are factors
> with two levels, then Regulated + Light + Nitrogen + Phosphorus +
> Nitrogen:Phosphorus requires 5 parameters. Add 1 for the random effect
> and you have 6 parameters or at least 60 observations. So this model
> might work with N = 66. However you will need to carefully check the
> model.
> 
> Best regards,
> 
> ir. Thierry Onkelinx
> Statisticus / Statistician
> 
> Vlaamse Overheid / Government of Flanders
> INSTITUUT VOOR NATUUR- EN BOSONDERZOEK / RESEARCH INSTITUTE FOR NATURE
> AND FOREST
> Team Biometrie & Kwaliteitszorg / Team Biometrics & Quality Assurance
> thierry.onkelinx at inbo.be
> Havenlaan 88 bus 73, 1000 Brussel
> www.inbo.be
> 
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> 
> 
> 
> 2018-04-04 15:55 GMT+02:00 John Ludlam <jludlam at fitchburgstate.edu>:
>> Hello,
>>
>> I have an experiment with six streams in two groups (regulated and control).  At each stream there were five sites (Transect).  At each site there were unreplicated nutrient treatments (N, P, N+P, C).  Light was measured at each site.
>>
>> Stream  Regulated       Transect        Nitrogen        Phosphorus      R       Light
>> Cranberry       Regulated       30      C       C       -0.102512563    2042.266667
>> Cranberry       Regulated       30      C       P       -0.08877551     2042.266667
>> Cranberry       Regulated       50      C       C       -0.107142857    1283.3
>> Cranberry       Regulated       50      N       C       -0.059375       1283.3
>> Cranberry       Regulated       70      C       C       -0.067346939    1336.6
>> Cranberry       Regulated       70      N       C       -0.063636364    1336.6
>> ...
>>
>> I would like to know if the response differs among groups (regulated vs control) or is related to light or nutrient treatment.  I have two separate analyses, N = 107 and N = 66 with different numbers of missing values (N = 120 before missing values).
>>
>> I think the appropriate model structure is:
>>
>> lme(Response ~ Regulated + Light + Nitrogen + Phosphorus + Nitrogen:Phosphorus), random=~1|Stream/Transect, data=data, method="ML"))
>>
>> However, I'm concerned that the model is far too complex for my sample size.  Any advice would be appreciated!
>>
>> Thanks!
>>
>> John P. Ludlam, Ph.D. - Fitchburg State University
>>
>>
>>
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