Hi Dani,
it would be better to start with a question you are trying to ask of your
data rather than trying to figure out what a particular function does.  with
your variables and model, even if the component terms were not significant,
they must in the model or the product of sunlight and aspect will NOT
represent the interaction.  also note that the tests of your components are
probably not what you think they are.  in general, tests of components of
interactions test the simple effect of that variable when the other variable
is 0.  hence, your 'significant' result for aspect pertains to when log
sunlight is 0, which probably isn't what you want to be testing.  what the
significant effect for sunlight means depends on how aspect was coded.  you
should check to see what code was used to know what zero means.

gary mcclelland
colorado

On Sun, Feb 10, 2008 at 6:40 AM, DaniWells <dani_wells210@hotmail.com>
wrote:

>
> Hello,
>
> I'm having some difficulty understanding the useage of the "dropterm()"
> function in the MASS library. What exactly does it do? I'm very new to R,
> so
> any pointers would be very helpful. I've read many definitions of what
> dropterm() does, but none seem to stick in my mind or click with me.
>
> I've coded everything fine for an interaction that runs as follows: two
> sets
> of data (one for North aspect, one for Southern Aspect) and have a
> logscale
> on the x axis, with survival on the y. After calculating my anova results
> i
> have all significant results (ie aspect = sig, logscale of sunlight = sig,
> and aspect:llight = sig).
>
> When i have all significant results in my ANOVA table, do i need
> dropterm(),
> or is that just to remove insignificant terms?
>
> Many thanks,
>
> Dani
> --
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