[R] Is this correct?

Rlover lmoto001 at fiu.edu
Mon Nov 24 03:45:55 CET 2008

I have to answer the following question for a homework assignment.

A researcher was interested in whether people taking part in sports at
university made more money after graduating, taking into account the
students' GPA. They sampled 200 alumni from a large university. The
variables are: income (income 10 years after graduating), sports (1 if they
did sports, 0 if they did not), and GPA (the grade point average at
university). Discuss the relationship between taking part in sports, GPA,
and income for these data. 

The R code I used so far is

Does sports predict GPA? 

> lm1<-lm(GPA~sports)
> summary(lm1)

Does sports predict income? 
> lm2<-lm(income~sports)
> summary(lm2)

Does GPA predict income? 
> lm3<-lm(income~GPA)
> summary(lm3)

Does sports predict income after accounting for GPA? 
> lm4<-lm(income~GPA+sports)
> summary(lm4)

Can someone let me know is the above is correct? I am not sure if to keep
all four regressions or only 1 and 4. Also, I need to plot the data on a
graph or table. Can anyone suggest how to do this? Thank you in advance :)

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