Compare slopes bewteen two datasets.

gistfile1.txt

# Make up some data
data1 <- data.frame(length = rnorm(20, 8), weight = 1:20)

data2 <- data.frame(length = rnorm(30, 8), weight = 1:30)


# add a dummy variable and combine the data 

data1$dummy <- 'data1'
data2$dummy <- 'data2'

combinedData <- rbind(data1, data2)


# Fit model that ignores the different data sets

model1 <- lm(length ~ weight, data = combinedData)

# Now fit an interaction between the dummy and the slope and coeficients. 
model2 <- lm(length ~ dummy*weight, data = combinedData)


library(lmtest)

lrtest(model1, model2)



# If you wanted to ONLY compare the slopes you could do

model3 <- lm(length ~ weight + dummy:weight, data = combinedData)
lrtest(model1, model3)