R help - Nov 2009 - linear trend line and a quadratic trend line.

Dear list users



How is it possible to visualise both a linear trend line and a quadratic trend
line on a plot
of two variables?



Here my almost working exsample.



data(Duncan)

attach(Duncan)

plot(prestige ~ income)

abline(lm(prestige ~ income), col=2, lwd=2)



Now I would like to add yet another trend line, but this time a quadratic one.
So I have two
trend lines. One linear trend line and a quadratic trend line. A trend line as
if I had taken
I(income^2) into the equation.



I know I can make two models and compare them using anova, but for pedagogical
resons I wold
like to visualise it. I know the trick from my past as an SPSS user, but I would
like to do
this in R as well. Se it in SPSS
http://www.childrens-mercy.org/stats/weblog2006/QuadraticRegression.asp



Thanks in adcance



Eric

Eric Fail wrote:
> 
> 
> Dear list users
> 
> 
> 
> How is it possible to visualise both a linear trend line and a quadratic
trend line on a plot
> of two variables?
> 
> 
> 
> Here my almost working exsample.
> 
> 
> 
> data(Duncan)
> 
> attach(Duncan)
> 
> plot(prestige ~ income)
> 
> abline(lm(prestige ~ income), col=2, lwd=2)
> 
> 
> 
> Now I would like to add yet another trend line, but this time a quadratic
one. So I have two
> trend lines. One linear trend line and a quadratic trend line. A trend line
as if I had taken
> I(income^2) into the equation.
> 
> 
> 
> I know I can make two models and compare them using anova, but for
pedagogical resons I wold
> like to visualise it. I know the trick from my past as an SPSS user, but I
would like to do
> this in R as well. Se it in SPSS
> http://www.childrens-mercy.org/stats/weblog2006/QuadraticRegression.asp
There's no precooked function that I am aware of, but the generic way is 
like

rg <- range(income)
N <- 200
x <- seq(rg[1], rg[2],, N)
pred <- predict(lm(prestige~ income+I(income^2)),
   newdata=data.frame(income=x))
lines(x, pred)

as usual, "like" means that if you can't be bothered with making
your
example reproducible, I can't be bothered with testing the code!


Well, actually, I found the Duncan data in library(car), so I did in 
fact test...

-- 
    O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
  (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45) 35327907

Here is a solution using ggplot2

n <- 50
Duncan <- data.frame(income = runif(n, 0, 50000))
Duncan$prestige <- with(Duncan, 0.01 * income - .0000001 * income ^2 +
rnorm(n, sd = 10))

library(ggplot2)
ggplot(Duncan, aes(y = prestige, x = income)) +
#define the data
		geom_point() +
#add the data points
		geom_smooth(method = "lm", colour = "red") +
#add a linear trend
		geom_smooth(method = "lm", formula = y ~ poly(x, 2),
color = "green") #add the quadratic trend

HTH,

Thierry


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens Eric Fail
Verzonden: zondag 8 november 2009 18:21
Aan: r-help at r-project.org
Onderwerp: [R] linear trend line and a quadratic trend line.




Dear list users



How is it possible to visualise both a linear trend line and a quadratic
trend line on a plot of two variables?



Here my almost working exsample.



data(Duncan)

attach(Duncan)

plot(prestige ~ income)

abline(lm(prestige ~ income), col=2, lwd=2)



Now I would like to add yet another trend line, but this time a
quadratic one. So I have two trend lines. One linear trend line and a
quadratic trend line. A trend line as if I had taken
I(income^2) into the equation.



I know I can make two models and compare them using anova, but for
pedagogical resons I wold like to visualise it. I know the trick from my
past as an SPSS user, but I would like to do this in R as well. Se it in
SPSS
http://www.childrens-mercy.org/stats/weblog2006/QuadraticRegression.asp



Thanks in adcance



Eric

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