R help - Sep 2012 - remove all terms with interaction factor in formula

Hi Folks,

I'm trying to find a way to remove all terms in a formula that contain a
particular interaction.

For example, in the formula below, I'd like to remove all terms that
contain the b:c interaction.
> attributes(terms( ~ a*b*c*d))$term.labels
 [1] "a"       "b"       "c"       "d"  
"a:b"     "a:c"
 [7] "b:c"     "a:d"     "b:d"     "c:d"
"a:b:c"   "a:b:d"
[13] "a:c:d"   "b:c:d"   "a:b:c:d"

My eventual use is to fit models with the reduced formulas.

For example:
> my_df = data.frame( iv = runif(100), a=runif(100), b=runif(100),
c=runif(100), d=runif(100))
> lm(iv ~ a*b*c*d, data=my_df)
I can remove particular terms with update(), but I don't see a way to
remove all terms that contain a given combination of factors.

Any help would be greatly appreciated.  Thanks!

Allie

~ a*b*d + a*c*d
-- Bert
On Thu, Sep 13, 2012 at 10:49 AM, Alexander Shenkin <ashenkin at ufl.edu>
wrote:
> Hi Folks,
>
> I'm trying to find a way to remove all terms in a formula that contain
a
> particular interaction.
>
> For example, in the formula below, I'd like to remove all terms that
> contain the b:c interaction.
>
>> attributes(terms( ~ a*b*c*d))$term.labels
>  [1] "a"       "b"       "c"      
"d"       "a:b"     "a:c"
>  [7] "b:c"     "a:d"     "b:d"    
"c:d"     "a:b:c"   "a:b:d"
> [13] "a:c:d"   "b:c:d"   "a:b:c:d"
>
> My eventual use is to fit models with the reduced formulas.
>
> For example:
>> my_df = data.frame( iv = runif(100), a=runif(100), b=runif(100),
> c=runif(100), d=runif(100))
>> lm(iv ~ a*b*c*d, data=my_df)
>
> I can remove particular terms with update(), but I don't see a way to
> remove all terms that contain a given combination of factors.
>
> Any help would be greatly appreciated.  Thanks!
>
> Allie
>
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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