self start for five-parameter logistic function
Self starter for a five-parameter logistic function.
logis5(x, asym1, asym2, xmid, iscal, theta) SSlogis5(x, asym1, asym2, xmid, iscal, theta)
x |
input vector (x) which is normally light intensity (PPFD, Photosynthetic Photon Flux Density). |
asym1 |
asymptotic value for low values of x |
asym2 |
asymptotic value for high values of x |
xmid |
value of x at which y = (asym1 + asym2)/2 |
iscal |
steepness of transition from asym1 to asym2 (inverse of the scale) |
theta |
asymmetry parameter, if it is equal to 1, this is the four parameter logistic |
This is known as the Richards' function or the log-logistic and it is described in Archontoulis and Miguez (2015) - (doi:10.2134/agronj2012.0506).
a numeric vector of the same length as x (time) containing parameter estimates for equation specified
logis5: vector of the same length as x (time) using the 5-parameter logistic
require(ggplot2) set.seed(1234) x <- seq(0, 2000, 100) y <- logis5(x, 35, 10, 800, 5, 2) + rnorm(length(x), 0, 0.5) dat <- data.frame(x = x, y = y) fit <- nls(y ~ SSlogis5(x, asym1, asym2, xmid, scal, theta), data = dat) ## plot ggplot(data = dat, aes(x = x, y = y)) + geom_point() + geom_line(aes(y = fitted(fit))) x <- seq(0, 2000) y <- logis5(x, 30, 10, 800, 5, 2) plot(x, y)
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