[R] Fitting linear models

[Dimitri Liakhovitski]

Try:
model1<-lm(PBW~SO4+NO3+NH4)
Does it work?
Dimitri

On Mon, Apr 20, 2009 at 7:26 PM, Vemuri, Aparna <avemuri at epri.com> 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.
>
> 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.
>
> Thanks
> Aparna
>
> model1<-lm(formula = PBW ~ SO4 + NH4 +NO3)
>> model1
>
> Call:
> lm(formula = PBW ~ SO4 + NH4 + NO3)
>
> Coefficients:
> (Intercept)          SO4      NH4          NO3
>    0.01323     -0.00430      0.06394           NA
>
>
>
>
>> model2<-lm(formula = PBW ~ SO4 + NO3 + Na +Cl  +NH4)
>> model2
>
> Call:
> lm(formula = PBW ~ SO4 + NO3 + Na + Cl + NH4)
>
> Coefficients:
> (Intercept)          SO4             NO3                        Na
> Cl                  NH4
>  -0.0006987    0.0196371   -0.0050303    0.0685020    0.0427431
> NA
>
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>
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>



-- 
Dimitri Liakhovitski
MarketTools, Inc.
Dimitri.Liakhovitski at markettools.com