TAPO (Thomas Agersten Poulsen) wrote:
> Dear R-list,
>
> It is not uncommon for laboratory equipment (e.g. spectrophotometers) to have a linear response in a certain interval and then go into saturation. I wonder if there is an R-function that models this; for instance by estimating the breakpoint and fitting a line below the breakpoint and a constant above.
>
> Best regards
> Thomas Poulsen
>
> --
> Thomas Poulsen Research Scientist. PhD, MSc
> Novozymes A/S Protein Design / Bioinformatics
> Brudelysvej 26, 1US.24 Phone: +45 44 42 27 23
> DK-2880 Bagsværd.
>
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I don't know if there is something this specific out there already, but you could use ?optim to do this. Here's a quick example:
set.seed(1) x <- rnorm(100) y <- ifelse(x < 0, 1 + 4 * x, 1) + rnorm(100, sd = 0.2)
brk <- function(par, x, y) {
a <- par[1] # intercept
b <- par[2] # slope
p <- par[3] # break point
h <- par[4] # height of saturation
yhat <- ifelse(x < p, a + b * x, h)
sum((y - yhat)^2)
}st <- coef(lm(y ~ x)) # starting values fit <- optim(c(st, 0, 0), brk, x = x, y = y) a <- fit$par[1] b <- fit$par[2] p <- fit$par[3] h <- fit$par[4]
yhat <- ifelse(x < p, a + b * x, h) plot(y ~ x) lines(x[order(x)], yhat[order(x)])
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