[R] Is this correct?

From: Rlover <lmoto001_at_fiu.edu>
Date: Sun, 23 Nov 2008 18:45:55 -0800 (PST)

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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