statforbiology (version 0.9.9)

expoGrowth: Exponential growth function

Description

These functions provide the exponential growth equation (expoGrowth), the exponential growth equation with self-starter for the nls function and the exponential growth equation with self-starter for the drm function in the drc package.

Usage

expoGrowth.fun(predictor, init, k)
  NLS.expoGrowth(predictor, init, k)
  DRC.expoGrowth(fixed = c(NA, NA), names = c("init", "k"))

Value

expoGrowth.fun() and NLS.expoGrowth() return a numeric value, while DRC.expoGrowth() returns a list containing the nonlinear function, the self starter function and the parameter names.

Arguments

predictor

a numeric vector of values at which to evaluate the model.

init

model parameter: response at predictor = 0

k

model parameter: growth rate

fixed

numeric vector. Specifies which parameters are fixed and at what value they are fixed. NAs for parameter that are not fixed.

names

a vector of character strings giving the names of the parameters. The default is reasonable.

Author

Andrea Onofri

Details

The exponential growth is given by the following function: $$ f(x) = \textrm{C0} \cdot \exp ( k x )\}$$

References

Ratkowsky, DA (1990) Handbook of nonlinear regression models. New York (USA): Marcel Dekker Inc.

Onofri, A. (2020). A collection of self-starters for nonlinear regression in R. See: https://www.statforbiology.com/2020/stat_nls_usefulfunctions/

Examples

library(statforbiology)
Time <- 1:20
Resp <- c(0.18, 0.64, 1.14, 0.67, 0.32, 0.86, 0.70, 0.73, 0.89, 0.48,
    2.20, 1.03, 1.14, 2.14, 1.31, 2.08, 1.85, 1.47, 1.98, 1.30)
model <- drm(Resp ~ Time, fct = DRC.expoGrowth())
summary(model)
model2 <- nls(Resp ~ NLS.expoGrowth(Time, a, b))
summary(model2)