SSL: Logistic equations

In statforbiology: Tools for Data Analyses in Biology

Logistic equations

Description

These functions provide the logistic equations with 4 (L4.fun), 3 (L3.fun) and 2 (L2.fun) parameters with self-starter for the nls function (NLS.L4, NLS.L3 and NLS.L2) and the self-starter for logistic function with two parameters for the drm function in the drc package (DRC.L2).

Usage

L4.fun(predictor, b, c, d, e)
L3.fun(predictor, b, d, e)
L2.fun(predictor, b, e)
NLS.L4(predictor, b, c, d, e)
NLS.L3(predictor, b, d, e)
NLS.L2(predictor, b, e)
DRC.L2(upper = 1, fixed = c(NA, NA), names = c("b", "e"))

Arguments

predictor	a numeric vector of values at which to evaluate the model
b	model parameter (slope at inflection point)
c	model parameter (lower asymptote)
d	model parameter (higher asymptote)
e	model parameter (abscissa at inflection point)
upper	numeric. For L.2, a upper asymptote different from 1 can be specified.
fixed	numeric vector. Specifies which parameters are fixed and at what value they are fixed. NAs for parameter that are not fixed.
names	names. A vector of character strings giving the names of the parameters. The default is reasonable.

Details

The logistic equation is parameterised as:

f(x) = c + \frac{d - c}{1+exp\left[-b(x - e)\right]}

for the 3- and 2-parameter model c is equal to 0, while for the 2-parameter model d is equal to 1.

Value

L4.fun, L3.fun, L2.fun, NLS.L4, NLS.L3 and NLS.L2 return a numeric value, while DRC.L2 returns a list containing the nonlinear function, the self starter function and the parameter names.

Author(s)

Andrea Onofri

Examples

data(beetGrowth)
mod3 <- nls(weightInf ~ NLS.L3(DAE, b, c, d), data = beetGrowth)
mod3b <- drm(weightInf ~ DAE, fct=DRC.L2(upper = 25), data = beetGrowth)

statforbiology documentation built on Oct. 30, 2024, 9:13 a.m.