On Wed, 11 Aug 2010, David Winsemius wrote:
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) ?
Quite so. It is just one way (and I am not touting it) of rescaling a
variable.
I
also wondered if he might need to be referred to:
?offset
??
?? back 'atcha.
My '??' was intended for the OP, but your guess seems good to me.
Chuck
--
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
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Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cbe...@tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
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