On Aug 29, 2009, at 7:56 AM, Markus Gesmann wrote:
Dear R-help,
Suppose I have the following data:
df=data.frame(x=1:10, y=c(1,2,3,4,5,12,14,16,18,20))
plot(y~x, df, t="b")
How can I fit a model which estimates the slopes between x = 1-5,
5-6, and 6-10?
Adding the factor f:
df$f <- gl(2,5)
Allows me to fit a linear model with interaction
lm(y ~ x:f, data=df)
which gives me the slope of 1 and 2 between 1-5, and 6-10
respectively, however it can not cope with the change from 5 to 6.
Can you be a bit more mathematically precise, ... and less
anthropomorphic, in explaining what you mean by "cannot cope"? When I
look at the plot of fitted values I fail to see any "coping"
deficiencies, at least with my understanding regarding what you were
expecting .... about which I am very much guessing at this point. Did
you want only two regression lines that share a joinpoint at x=5.5?
I would appreciate if someone could point me into the right direction.
Many thanks
Markus
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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