logist.fx: A function having the mathematical expression of the Logistic...
In biometrics: A Package for Biometrics and Modelling

logist.fx

R Documentation

A function having the mathematical expression of the Logistic model.

Description

Function of the Logistic model, based upon three parameters and a single predictor variable as follows

y_i= \frac{\alpha}{1+ \mathrm{e}^{\beta - \gamma x_i}},

where: y_i and x_i are the response and predictor variable, respectively, for the i-th observation; and the rest are parameters (i.e., coefficients).

Usage

logist.fx(x, alpha, beta, gamma, upsilon = 0)

Arguments

`x`	is the predictor variable.
`alpha`	is the coefficient-parameter `\alpha`.
`beta`	is the coefficient-parameter `\beta`.
`gamma`	is the coefficient-parameter `\gamma`.
`upsilon`	is an optional constant term that force the prediction of y when x=0. Thus, the new model becomes `y_i = \Upsilon+ f\left(x_i,\mathbf{\theta}\right)`, where `\mathbf{\theta}` is the vector of coefficients of the above described function represented by `f(\cdot)`. The default value for `\Upsilon` is 0.

Value

Returns the response variable based upon the predictor variable and the coefficients.

Author(s)

Christian Salas-Eljatib.

References

Pearl R. 1909. Some recent studies on growth. The American Naturalist 43(509):302-316.
Salas-Eljatib C, Mehtatalo L, Gregoire TG, Soto DP, Vargas-Gaete R. 2021. Growth equations in forest research: mathematical basis and model similarities. Current Forestry Reports 7:230-244. \Sexpr[results=rd]{tools:::Rd_expr_doi("10.1007/s40725-021-00145-8")}
Salas-Eljatib C. 2025. Funciones alométricas: reparametrizaciones y características matemáticas. Documento de trabajo No. 1, Serie: Cuadernos de biometría, Laboratorio de Biometría y Modelación Forestal, Universidad de Chile. Santiago, Chile. 51 p. https://biometriaforestal.uchile.cl

Examples

# Predictor variable values to be used
time<-seq(5,60,by=0.01)
# Using the function
y<-logist.fx(x=time,alpha=22,beta=1.4,gamma=.1)
plot(time,y,type="l")
#'

biometrics documentation built on March 20, 2026, 5:09 p.m.