Hi,
I read almost all I found in prior R-Help list about How to compare
two regression line slopes.
So, I made a didactic example to illustrate a solution cited by Ben Bolker:
===============================================
Subject: Re: [R] How to compare two regression line slopes
From: Ben Bolker (bol...@ufl.edu)
Date: Jan 27, 2009 1:52:20 pm
List: org.r-project.r-help
===============================================
I would like to know the opinion of other specialists.
I'm needing do apply this solution in a real (and experimental)
situation and the
data are "like" iris data.
# libraries
library(ggplot2)
# a example based in iris data (nonsense)
qplot(Petal.Width, Sepal.Length,
data=iris,
facets= . ~ Species,
geom=c('point', 'smooth'),
se=F,
method='lm',
formula=y ~ x)
# Function to test slope
t.slope <- function(dF) {
fit <- lm(y~x*f, data=dF)
coef(summary(fit))["x:f2",]
}
# setosa vs. versicolor
x <- subset(iris, Species != 'virginica', sel=Petal.Length)[,]
y <- subset(iris, Species != 'virginica', sel=Petal.Width)[,]
f <- factor(rep(1:2, each=50))
dF <- data.frame(x, y, f)
t.slope(dF)
# setosa vs. virginica
x <- subset(iris, Species != 'versicolor', sel=Petal.Length)[,]
y <- subset(iris, Species != 'versicolor', sel=Petal.Width)[,]
f <- factor(rep(1:2, each=50))
dF <- data.frame(x, y, f)
t.slope(dF)
# versicolor vs. virginica
x <- subset(iris, Species != 'setosa', sel=Petal.Length)[,]
y <- subset(iris, Species != 'setosa', sel=Petal.Width)[,]
f <- factor(rep(1:2, each=50))
dF <- data.frame(x, y, f)
t.slope(dF)
Thanks,
--
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Jose Claudio Faria
Estatistica - prof. Titular
UESC/DCET/Brasil
joseclaudio.fa...@gmail.com
joseclaudio.fa...@terra.com.br
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