logistic4_fn {drda}

R Documentation

4-parameter logistic function

Description

Evaluate at a particular set of parameters the 4-parameter logistic function.

Usage

logistic4_fn(x, theta)

Arguments

x	numeric vector at which the logistic function is to be evaluated.
theta	numeric vector with the four parameters in the form c(alpha, beta, eta, phi).

Details

The 4-parameter logistic function ⁠f(x; theta)⁠ is defined here as

alpha + (beta - alpha) / (1 + exp(-eta * (x - phi)))

where theta = c(alpha, beta, eta, phi), alpha is the lower horizontal asymptote, beta > alpha is the upper horizontal asymptote, eta is the steepness of the curve or growth rate (also known as the Hill coefficient), and phi is the value of x at which the curve is equal to its mid-point.

Value

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

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