[R] a question regarding updating formulas with coefficients

[David Winsemius]

On Aug 11, 2010, at 6:45 PM, Charles C. Berry wrote:

> On Wed, 11 Aug 2010, David Winsemius wrote:
>
>>
>> On Aug 11, 2010, at 6:03 PM, Jarrett Byrnes wrote:
>>
>>> I have formulae with coefficents that I would like to update.   
>>> However, I get some strange results.  For example, see the  
>>> following:
>>> For the formula y ~ d+ 3*r+t
>>
>> Did you really get meaningful results from that formula? Care to  
>> provide an example? Maybe there is something more for me to learn.
>>
>>
>>> I want to add a variable p, so
>>> > update(y~d+0*r+t, .~.+p)
>>
>> In formulas the "*" operator is an interaction creator. so you told  
>> R to make 0 + r + 0:r. Probably not what you thought you were  
>> doing. So what were you trying to do anyway?
>>
>>> produces
>>> y ~ d + t + p - 1
>>
>> Which at least explains why you got the -1 (which in R formulas is  
>> that same as +0).
>>> If the coefficient is not 0, but rather, something else - say, 3,
>>
>> What do you think you are accomplishing when you put a scalar  
>> coefficient in the formula?
>>
>
> Maybe he wants
>
> 	?I

Possibly, but wouldn't that just result in a "deflation" by a factor  
of 3 of the estimated coefficient if you wrappedI around that rem ...   
I(3*r) ? I also wondered if he might need to be referred to:

?offset


>
> ??

?? back 'atcha.

-- 
David.
>
> Chuck
>
>>> I get the following:
>>> > update(y~d+3*r+t, .~.+p)
>>> Error in terms.formula(tmp, simplify = TRUE) :
>>> invalid model formula in ExtractVars
>>> > Is there a way to do this,
>>
>>> or a different call I should be trying?
>>
>> What you should be doing depends on what you want to happen.
>>
>> -- 
>>
>> David
>

David Winsemius, MD
West Hartford, CT