[R-sig-eco] Help with model selection

[Bob OHara]

The design is good. But the details will matter. I think the best advice 
I can offer is to seek advice! Your university has a consulting service: 
http://www.stat.ualberta.ca/~tcc/#home, and I think half an hour with 
them looking at your data with you will help a lot.

You have a multivariate response, so a MANOVA is probably needed. The 
full (horrible) model you have is

Time*Treatment*Moisture/Site/Tree

where Time is before/after, and Treatment is the inoculation. Hopefully 
you can remove some of the interactions: the Time*Treatment*Moisture 
terms are important, so you would hope that the interactions with Tree 
and Site don't matter.

Tree should probably be a random effect, although it is less important 
her because your design is good.

I think giving more advice than this would require some poking around 
the data, but I hope this is a help.

Bob

On 03/16/2018 03:33 AM, Altaf Hussain wrote:
> Hello everyone,
>
> I have some field collected data from *90* random trees on *6* sites and
> I am comparing the trees for the concentrations of different chemical
> compounds. The chemicals are of two different types, *(Group 1 = A, B, C,
> D, E, F and their sum, X)* & *(Group 2 = G, H, I, J and their sum, Y)*. I
> also have the sum of X and Y that I call Z. All variables are continuous
> and use the same units. The 6 sites are closely located but are unique in
> terms of their soil nutrients (N, P K, S and their sum, Tot.Nutrients,
> *continuous/same
> units*) and soil moisture. I further grouped the sites for soil moisture
> and I now have 2 Dry, 2 Medium dry and 2 Wet sites, (6/3=2), character
> variables. Each site has 15 trees and an equal number of trees i.e., (5)
> were treated using different fungal inoculation densities i.e., (0 for
> control, 4 and 16).  The trees were sampled in a pre and post inoculation
> manner, where the pre inoculation samples serve to determine the baseline
> chemistry of trees and the post inoculation samples will show the chemical
> responses of the trees to inoculations *(if any!).*
>
> Alright, first thing first, I admit that I am very new to statistical
> modeling, which is why I have such a complicated design, and which is why I
> am posting ☺.  The following are the questions that I want to answer using
> my dataset introduced above:
>
> 1) How do I compare my 6 sites for trees' chemical responses to the
> different inoculation densities?
> 2) How do I check if any of the nutrients, their interaction, or their
> interaction with soil moisture is playing a role in the chemical variation?
>
> I am open to any suggestions that will improve my study!
>
> I am using RStudio version 3.4.3 for the analysis.
>
> I appreciate any input that will help me with the analysis.
>
> Regards,
>
> //Simon
>
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>
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-- 
Bob O'Hara
Institutt for matematiske fag
NTNU
7491 Trondheim
Norway

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