On Apr 20, 2009, at 7:26 PM, Vemuri, Aparna wrote:
I am not sure if this is an R-users question, but since most of you
here
are statisticians, I decided to give it a shot.
You can omit the unnecessary preambles.
I am using the lm() function in R to fit a dependent variable to a set
of 3 to 5 independent variables. For this, I used the following
commands:
model1<-lm(function=PBW~SO4+NO3+NH4)
Coefficients:
(Intercept) SO4 NO3 NH4
0.01323 0.01968 0.01856 NA
and
model2<-lm(function=PBW~SO4+NO3+NH4+Na+Cl)
Coefficients:
(Intercept) SO4 NO3 NH4
Na Cl
-0.0006987 -0.0119750 -0.0295042 0.0842989 0.1344751
NA
In both cases, the last independent variable has a coefficient of NA
in
the result. I say last variable because, when I change the order of
the
variables, the coefficient changes (see below). Can anyone point me to
the reason R behaves this way? Is there anyway for me to force R to
use
all the variables? I checked the correlation matrices to makes sure
there is no orthogonality between the variables.
You really did not name your dependent variable "function" did you?
Please stop that.
Just a guess, ... since you have not provided enough information to do
otherwise, ... Are all of those variables 1/0 dummy variables? If so
and if you want to have an output that satisfies your need for
labeling the coefficients as you naively anticipate, then put "0+" at
the beginning of the formula or "-1" at the end, so that the intercept
will disappear and then all variables will get labeled as you expect.
--
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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