David, Thanks for the suggestions. No, I did not label my dependent variable "function".
My dependent variable PBW and all the independent variables are continuous variables. It is especially troubling since the order in which I input independent variables determines whether or not it gets a coefficient. Like I already mentioned, I checked the correlation matrix and picked the variables with moderate to high correlation with the independent variable. . So I guess it is not so naïve to expect a regression coefficient on all of them. Dimitri model1<-lm(PBW~SO4+NO3+NH4), gives me the same result as before. Bert: This is not homework. But I will remember to do my research before posting here. Aparna -----Original Message----- From: David Winsemius [mailto:dwinsem...@comcast.net] Sent: Monday, April 20, 2009 5:35 PM To: Vemuri, Aparna Cc: r-help@r-project.org Subject: Re: [R] Fitting linear models 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 ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
