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 Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalga...@biostat.ku.dk) FAX: (+45) 35327907
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