maxLik (version 1.4-6)

maxLik-package: Maximum Likelihood Estimation

Description

This package contains a set of functions and tools for Maximum Likelihood (ML) estimation. The focus of the package is on the non-linear optimization from the ML viewpoint, and it provides several convenience wrappers and tools, like BHHH algorithm, variance-covariance matrix and standard errors.

Details

maxLik package is a set of convenience tools and wrappers focusing on Maximum Likelihood (ML) analysis, but it also contains tools for other optimization tasks. The package includes a) wrappers for several existing optimizers (implemented by optim); b) original optimizers, including Newton-Raphson and Stochastic Gradient Ascent; and c) several convenience tools to use these optimizers from the ML perspective. Examples are BHHH optimization (maxBHHH) and utilities that extract standard errors from the estimates. Other highlights include a unified interface for all included optimizers, tools to test user-provided analytic derivatives, and constrained optimization.

A good starting point to learn about the usage of maxLik are Henningsen & Toomet (2011), an introductory paper to the package, and the included vignette “Stochastic Gradient Ascent in maxLik”. Use vignette(package="maxLik") to see the available vignettes, and vignette("stochastic-gradient-maxLik") to read the stochastic gradient ascent vignette.

From the user's perspective, the central function in the package is maxLik. In its simplest form it takes two arguments: the log-likelihood function, and a vector of initial parameter values (see the example below). It returns an object of class ‘maxLik’ with convenient methods such as summary, coef, and stdEr. It also supports a plethora of other arguments, for instance one can supply analytic gradient and Hessian, select the desired optimizer, and control the optimization in different ways.

A useful utility functions in the package is compareDerivatives that allows one to compare the analytic and numeric derivatives for debugging purposes. Another useful function is condiNumber for analyzing multicollinearity problems in the estimated models.

In the interest of providing a unified user interface, all the optimizers are implemented as maximizers in this package. This includes the optim-based methods, such as maxBFGS and maxSGA, the maximizer version of popular Stochastic Gradient Descent.

References

Henningsen A, Toomet O (2011). “maxLik: A package for maximum likelihood estimation in R.” Computational Statistics, 26(3), 443-458. doi: 10.1007/s00180-010-0217-1.

Examples

# NOT RUN {
### estimate mean and variance of normal random vector

## create random numbers where mu=1, sd=2
set.seed(123)
x <- rnorm(50, 1, 2 )

## log likelihood function.
## Note: 'param' is a 2-vector c(mu, sd)
llf <- function(param) {
   mu <- param[1]
   sd <- param[2]
   llValue <- dnorm(x, mean=mu, sd=sd, log=TRUE)
   sum(llValue)
}

## Estimate it with mu=0, sd=1 as start values
ml <- maxLik(llf, start = c(mu=0, sigma=1) )
print(summary(ml))
## Estimates close to c(1,2) :-)
# }