[R] when to use "I", "as is" caret

[Uwe Ligges]

On 14.09.2012 09:41, agent dunham wrote:
> Dear community,
>
> I've check it while working, but just to reassure myself.  Let's say we have
> 2 models:
>
> model1 <-  lm(vdep ~ log(v1) + v2 + v3 + I(v4^2) , data = mydata)
> model2 <-   lm(vdep ~ log(v1) + v2 + v3 + v4^2, data = mydata)
>
> So in model1 you really square v4; and in model2,  v4*^2 *doesn't do
> anything, does it? Model2 could be rewritten:
> model2b <-   lm(vdep ~ log(v1) + v2 + v3 + v4, data = mydata) and nothing
> changes, doesn't it?
>
> This "I" caret is essential with powering or when including transformations
> as I(1/(v2+v3)) but not with log transformation, isn't it?. Is there any
> other transformation where I muss use also this "I", as is caret?


You need it whenever you are using operators with a special meaning in 
formulas such as "+", "-", "*", "/", "|", "^", ":" etc.

v4^2 means: Take the variables v4 and all their two-way interactions. 
Since v4 is singular, there are no two-way interactiosn available and it 
is not changed.

Best,
Uwe Ligges




> Thanks in advance,
> user at host.com
>
>
>
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