[R] help interpreting a model summary

[David Winsemius]

On Sep 19, 2010, at 5:59 PM, zozio32 wrote:

>
> Thanks for you're long answer.
> I have to say, I am not fully sure of what you're meaning  
> everywhere. As I
> said, I am merely following a recipe book, and when things depart  
> from it I
> am a bit lost.
>
> I'll try to answer to each of your paragraphs:
> 3:  I was not wanting to include 3-way interactions, but that's the  
> only
> way I found to include a 2 way interaction in my piecewise linear  
> model.

Yes. That makes sense to me.

> I could obviously include only angleNoise*reflection, but I thought  
> that was
> not very consistent with the fact that reflection variable was split  
> in 2.
> I could may be define the point of separation, and then create 2  
> separate
> models in the form "lm(weightedDiff ~ angleNoise*reflection)".

Yes, I see that you did, and that may be helpful in understanding the  
estimates. The question I would ask you is why you are putting a  
breakpoint in a physical model. Unless there is a phase change or some  
discontinuity in effects at that point, I think the breakpoint looks  
artificial. Is "refelction" somehow physically connected with the  
breakpoint? Some sort of acoustic timing phenomenom? Or reflected wave  
effect in a hydraulic model?

> I merely
> thought that my formulation was a way to combine them together.
> Basically, i am expecting both parameters to degrade my signal, but  
> I'll not
> be surprised if passed a certain level of reflection, having noise  
> or not in
> my angles is not really relevant, hence the interaction parameter. The
> piecewise linear model is a way to take into account the curvature  
> in the
> data that I can observe on a straight scatter plot.
>
> 1:  Thanks for the first part, i think I can make sens of it. ;)
> I guess I can ignore this parameter in that case.  By the way, which  
> type of
> Anova you refering to: creating a factor with "high" and "low" level  
> of
> interation, and fitting the interation between angleNoise and this new
> factor?

If you took a model with all of the data and fit first a model without  
the break point and one with the breakpoint and then looked at the  
output of anova(model1) and annova(model2) the difference in deviance  
across the two models is distributed (asymptotically anyway) as a chi- 
square statistic with the difference in number of degrees of freedom.  
That's a much better basis for deciding whether the addition of the  
term is "statistically significant".
>
> 2: first, i was mislead by the meaning of this factor. i only  
> encounter the
> version were it's "TRUE", not "FALSE" which is the difference.
> I think I also use "important" in a wrong way. I should have used
> "significant" instead.

You had specified a model that that terms with both (reflection >=  
Break[xMin]) and (reflection < Break[xMin]) and the lm program  threw  
away all the levels with reflection >= Break[xMin]. If you had only  
specified the the model with only reflection >= Break[xMin] you would  
have gotten an identical model as far as predictions were concerned,  
but the signs would have been reversed for any of the levels with the  
inequality term in them.

> After, i have to admit that I am lost  when you're talking about  
> models with
> reversed inequality...
>
> 4. Not much to say here, i knew they were pointless but the results  
> from my
> formulation of the model. I don't thnik there is a need to removed  
> them no?

Not really. And I did not mean to say they were meaningless... as long  
as there is some physical meaning that motivated the exercise. I was  
chiding you for making constructions that you did not know how to  
interpret. If this were a consulting session I would have stopped you  
at an early point and tried to understand what the physical system  
"looked like", ie., what the measurements represented, and where the  
current state of knowledge about them stands.
>
>
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David Winsemius, MD
West Hartford, CT