Generalised logistic function


The generalised logistic function or curve, also known as Richards' curve, originally developed for growth modelling, is an extension of the logistic or sigmoid functions, allowing for more flexible S-shaped curves:
where = weight, height, size etc., and = time.
It has five parameters:
The equation can also be written:
where can be thought of as a starting time,
Including both and can be convenient:
this representation simplifies the setting of both a starting time and the value of Y at that time.
The general model is sometimes named a "Richards' curve" after F. J. Richards, who proposed the general form for the family of models in 1959.
The logistic, with maximum growth rate at time, is the case where.

Generalised logistic differential equation

A particular case of the generalised logistic function is:
which is the solution of the Richards' differential equation :
with initial condition
where
provided that ν > 0 and α > 0.
The classical logistic differential equation is a particular case of the above equation, with ν =1, whereas the Gompertz curve can be recovered in the limit provided that:
In fact, for small ν it is
The RDE models many growth phenomena, including the growth of tumours. In oncology its main biological features are similar to those of the Logistic curve model.
The RDE models are widely used to describe infection trajectory in epidemiological modeling; see for COVID-19 application.

Gradient of generalized logistic function

When estimating parameters from data, it is often necessary to compute the partial derivatives of the logistic function with respect to parameters at a given data point . For the case where,

Special cases

The following functions are specific cases of Richards' curves: