logistic2_fn {drda}R Documentation

2-parameter logistic function


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


logistic2_fn(x, theta)



numeric vector at which the logistic function is to be evaluated.


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.


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

⁠g(x; theta) = 1 / (1 + exp(-eta * (x - phi)))⁠ ⁠f(x; theta) = alpha + delta g(x; theta)⁠

where theta = c(alpha, delta, eta, phi) and eta > 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.


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

[Package drda version 2.0.3 Index]