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

strandg.fx

R Documentation

A function having the mathematical expression of the Strand model.

Description

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

y_i= \left(\frac{x_i}{\alpha +\beta x_i}\right)^{\gamma},

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

strandg.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(x_i,\mathbf{\theta})`, 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

Strand L. 1964. Numerical constructions of site-index curves. Forest Sci. 10(4):410-414.
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<-strandg.fx(x=time,alpha=0.98,beta=0.52,gamma=4.83)
plot(time,y,type="l")

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