Hi,
I'm a new member here in the list. I am a graduate from University of Georgia. Recently in doing analysis using lme on a dataset, I found several questions:
1. How to express the equation when the correlation structure is very complicated. For exmaple, if the fixed is y(t)=0.03x1(t)+1.5x2(t)(I omitted "hat" and others). And the model with corARMA(p=2,q=3) is proper. What will be the complete equation?
2. Is is that any regression error will be stationary? (Forgive me for my poor math background. This may be a simple question to most people.) Since corARIMA is not available.
3. Why not make a function to automatically select the best corARMA structure (setting max p and q and the computer takes care of the rest)?
4. When the initial model (without considering correlation structure) has many variables and some have no significance, should I use stepAIC first to eliminate some variables? Or, try corARMA with different combination of p and q, which may make more variables significant without having to reduce variable. I prefer the latter. 
5. If the best corARMA model out of a model still contain some insignificant variables, I would use drop1 (instead of stepAIC) and then try all the possible corARMA structures again. So my steps would be drop1, corARMA, drop1,corARMA, til I get a model with all variables significant. If the initial model has many variables, it would be a time-consuming process. Is it proper to do so? If proper, it'll be wonderful if a new function is developed to automatically do so.
Thanks, Hanhan



Xianglu Han

206 Environmental Health Science

University of Georgia 30602

Phone: 706 255 2308

 



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