Hey, all. I'm working on a data set with a broken stick linear regression where I know one of the two slopes. It is a negative linear function until the line intersects with the x-axis, at which point it becomes 0. It is not a nonlinear asymptotic function, and, indeed, using negative exponential or logistic types of fits as an approximation has tended to lead to an under or overestimation of values. I am also very interested to know just what the breakpoint in the data is.
Essentially if x<psi y = a + bx + error, where b is negative else y=0+error and I want to know the value of psi, as well as a and b. The error is also most likely gamma distributed, as values<0 are not possible (nutrient concentrations). I have attempted to use the segmented package, specifying something close to the visually estimated breakpoint as the value of psi, but it continues to return fits with a breakpoint that occurs somewhere in the middle of the linear portion of the line, and the slope and intercept of the second half of the regression is not 0. Is there either a package that exists that will allow me to estimate such a model, or a function that I can use for optim or nlm (I admit, I am a rank novice at coding such functions). Thanks so much! -Jarrett ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.