loglogistic2_fn: 2-parameter log-logistic function

In drda: Dose-Response Data Analysis

2-parameter log-logistic function

Description

Evaluate at a particular set of parameters the 2-parameter log-logistic function.

Usage

loglogistic2_fn(x, theta)

Arguments

x	numeric vector at which the function is to be evaluated.
theta	numeric vector with the four parameters in the form c(alpha, delta, eta, phi). alpha can only be equal to 0 or 1 while delta can only be equal to 1 or -1.

Details

The 2-parameter log-logistic function ⁠f(x; theta)⁠ is defined here as

⁠g(x; theta) = x^eta / (x^eta + phi^eta)⁠ ⁠f(x; theta) = alpha + delta g(x; theta)⁠

where x >= 0, theta = c(alpha, delta, eta, phi), eta > 0, and phi > 0. Only eta and phi are free to vary (therefore the name) while vector c(alpha, delta) is constrained to be either c(0, 1) (monotonically increasing curve) or c(1, -1) (monotonically decreasing curve).

This function allows values other than ⁠{0, 1, -1}⁠ for alpha and delta but will coerce them to their proper constraints.

Value

Numeric vector of the same length of x with the values of the log-logistic function.

drda documentation built on April 3, 2025, 6 p.m.