statforbiology (version 0.9.9)

SSGompertz: Gompertz equations

Description

These functions provide the Gompertz equations with 4 (G4.fun), 3 (G3.fun) and 2 (G2.fun) parameters with self-starter for the nls function (NLS.G4, NLS.G3 and NLS.G2).

Usage

G4.fun(predictor, b, c, d, e)
G3.fun(predictor, b, d, e)
G2.fun(predictor, b, e)
NLS.G4(predictor, b, c, d, e)
NLS.G3(predictor, b, d, e)
NLS.G2(predictor, b, e)

Value

All these functions return a numeric value.

Arguments

predictor

a numeric vector of values at which to evaluate the model

b

model parameter (slope at inflection point)

c

model parameter (lower asymptote)

d

model parameter (higher asymptote)

e

model parameter (abscissa at inflection point)

Author

Andrea Onofri

Details

The Gompertz equation is parameterised as: $$ f(x) = c + (d - c) \, \exp \left[-exp(-b (x - e))\right] $$ It is a sygmoidally shaped curve and it is asymmetric about its inflection point. For the 3- and 2-parameter model c is equal to 0, while for the 2-parameter model d is equal to 1.

Examples

data(beetGrowth)
mod3 <- nls(weightInf ~ NLS.G3(DAE, b, c, d), data = beetGrowth)
summary(mod3)
plot(mod3)