Question

I am using the R package segmented to calculate parameters for a model, in which the response variable is linearly correlated with the explanatory variable until a breakpoint, then the response variable becomes independent from the explanatory variable. In other words, a segmented linear model with the second part having a slope = 0.
What I already did is:

linear1 <- lm(Y ~ X)
linear2  <- segmented (linear1, seg.Z = ~ X, psi = 2)

This gives a model that have a very good first line, but the second line is not horizontal (but not significant). I want to make the second line horizontal. (psi = 2 is the place where I observed a breakpoint.)

Also, when I use “abline” to show the broken line on the plotting, it only show the first part of the model, giving a warning: “only using the first two of 4 regression coefficients”. How could I display both parts of the model?

To input my data into R:

X <- c(0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0)
Y <- c(1.31, 1.60, 1.86, 2.16, 2.44, 2.71, 3.00, 3.24, 3.57, 3.81, 3.80, 3.83, 3.78, 3.94, 3.75, 3.89)

Answer 1

This is as easy as using the plot method for segmented class objects provided by the package segmented and linked in the help for segmented

Assuming your data is in the data.frame d

linear2  <- segmented (linear1, seg.Z = ~ X, psi = 2, data = d)
plot(linear2)
points(Y~X, data = d)

An easy way to fudge a horizontal line would be to replace the coefficient with value required for that line to be horizontal

fudgedmodel <- linear2
fudgedmodel$coefficients[3] <- - fudgedmodel$coefficients[2]
plot(fudgedmodel)
points(Y~X, data = d)