minpack.lm (version 1.2-4)

nls.lm: Addresses NLS problems with the Levenberg-Marquardt algorithm

Description

The purpose of nls.lm is to minimize the sum square of the vector returned by the function fn, by a modification of the Levenberg-Marquardt algorithm. The user may also provide a function jac which calculates the Jacobian.

Usage

nls.lm(par, lower=NULL, upper=NULL, fn, jac = NULL,
       control = nls.lm.control(), ...)

Value

A list with components:

par

The best set of parameters found.

hessian

A symmetric matrix giving an estimate of the Hessian at the solution found.

fvec

The result of the last fn evaluation; that is, the residuals.

info

info is an integer code indicating the reason for termination.

0

Improper input parameters.

1

Both actual and predicted relative reductions in the sum of squares are at most ftol.

2

Relative error between two consecutive iterates is at most ptol.

3

Conditions for info = 1 and info = 2 both hold.

4

The cosine of the angle between fvec and any column of the Jacobian is at most gtol in absolute value.

5

Number of calls to fn has reached maxfev.

6

ftol is too small. No further reduction in the sum of squares is possible.

7

ptol is too small. No further improvement in the approximate solution par is possible.

8

gtol is too small. fvec is orthogonal to the columns of the Jacobian to machine precision.

9

The number of iterations has reached maxiter.

message

character string indicating reason for termination

.

diag

The result list of diag. See Details.

niter

The number of iterations completed before termination.

rsstrace

The residual sum of squares at each iteration. Can be used to check the progress each iteration.

deviance

The sum of the squared residual vector.

Arguments

par

A list or numeric vector of starting estimates. If par is a list, then each element must be of length 1.

lower

A numeric vector of lower bounds on each parameter. If not given, the default lower bound for each parameter is set to -Inf.

upper

A numeric vector of upper bounds on each parameter. If not given, the default upper bound for each parameter is set to Inf.

fn

A function that returns a vector of residuals, the sum square of which is to be minimized. The first argument of fn must be par.

jac

A function to return the Jacobian for the fn function.

control

An optional list of control settings. See nls.lm.control for the names of the settable control values and their effect.

...

Further arguments to be passed to fn and jac.

Details

Both functions fn and jac (if provided) must return numeric vectors. Length of the vector returned by fn must not be lower than the length of par. The vector returned by jac must have length equal to \(length(\code{fn}(\code{par}, \dots))\cdot length(\code{par})\).

The control argument is a list; see nls.lm.control for details.

Successful completion.

The accuracy of nls.lm is controlled by the convergence parameters ftol, ptol, and gtol. These parameters are used in tests which make three types of comparisons between the approximation \(par\) and a solution \(par_0\). nls.lm terminates when any of the tests is satisfied. If any of the convergence parameters is less than the machine precision, then nls.lm only attempts to satisfy the test defined by the machine precision. Further progress is not usually possible.
The tests assume that fn as well as jac are reasonably well behaved. If this condition is not satisfied, then nls.lm may incorrectly indicate convergence. The validity of the answer can be checked, for example, by rerunning nls.lm with tighter tolerances.

First convergence test.
If \(|z|\) denotes the Euclidean norm of a vector \(z\), then this test attempts to guarantee that $$|fvec| < (1 + \code{ftol})\,|fvec_0|,$$ where \(fvec_0\) denotes the result of fn function evaluated at \(par_0\). If this condition is satisfied with ftol \(\simeq 10^{-k}\), then the final residual norm \(|fvec|\) has \(k\) significant decimal digits and info is set to 1 (or to 3 if the second test is also satisfied). Unless high precision solutions are required, the recommended value for ftol is the square root of the machine precision.

Second convergence test.
If \(D\) is the diagonal matrix whose entries are defined by the array diag, then this test attempt to guarantee that $$|D\,(par - par_0)| < \code{ptol}\,|D\,par_0|,$$ If this condition is satisfied with ptol \(\simeq 10^{-k}\), then the larger components of \((D\,par)\) have \(k\) significant decimal digits and info is set to 2 (or to 3 if the first test is also satisfied). There is a danger that the smaller components of \((D\,par)\) may have large relative errors, but if diag is internally set, then the accuracy of the components of \(par\) is usually related to their sensitivity. Unless high precision solutions are required, the recommended value for ptol is the square root of the machine precision.

Third convergence test.
This test is satisfied when the cosine of the angle between the result of fn evaluation \(fvec\) and any column of the Jacobian at \(par\) is at most gtol in absolute value. There is no clear relationship between this test and the accuracy of nls.lm, and furthermore, the test is equally well satisfied at other critical points, namely maximizers and saddle points. Therefore, termination caused by this test (info = 4) should be examined carefully. The recommended value for gtol is zero.

Unsuccessful completion.

Unsuccessful termination of nls.lm can be due to improper input parameters, arithmetic interrupts, an excessive number of function evaluations, or an excessive number of iterations.

Improper input parameters.
info is set to 0 if \(length(\code{par}) = 0\), or \(length(fvec) < length(\code{par})\), or ftol \(< 0\), or ptol \(< 0\), or gtol \(< 0\), or maxfev \(\leq 0\), or factor \(\leq 0\).

Arithmetic interrupts.
If these interrupts occur in the fn function during an early stage of the computation, they may be caused by an unacceptable choice of \(par\) by nls.lm. In this case, it may be possible to remedy the situation by rerunning nls.lm with a smaller value of factor.

Excessive number of function evaluations.
A reasonable value for maxfev is \(100\cdot (length(\code{par}) + 1)\). If the number of calls to fn reaches maxfev, then this indicates that the routine is converging very slowly as measured by the progress of \(fvec\) and info is set to 5. In this case, it may be helpful to force diag to be internally set.

Excessive number of function iterations.
The allowed number of iterations defaults to 50, can be increased if desired.

The list returned by nls.lm has methods for the generic functions coef, deviance, df.residual, print, residuals, summary, confint, and vcov.

References

J.J. Moré, "The Levenberg-Marquardt algorithm: implementation and theory," in Lecture Notes in Mathematics 630: Numerical Analysis, G.A. Watson (Ed.), Springer-Verlag: Berlin, 1978, pp. 105-116.

See Also

optim, nls, nls.lm.control

Examples

###### example 1

## values over which to simulate data 
x <- seq(0,5,length=100)

## model based on a list of parameters 
getPred <- function(parS, xx) parS$a * exp(xx * parS$b) + parS$c 

## parameter values used to simulate data
pp <- list(a=9,b=-1, c=6) 

## simulated data, with noise  
simDNoisy <- getPred(pp,x) + rnorm(length(x),sd=.1)
 
## plot data
plot(x,simDNoisy, main="data")

## residual function 
residFun <- function(p, observed, xx) observed - getPred(p,xx)

## starting values for parameters  
parStart <- list(a=3,b=-.001, c=1)

## perform fit 
nls.out <- nls.lm(par=parStart, fn = residFun, observed = simDNoisy,
xx = x, control = nls.lm.control(nprint=1))

## plot model evaluated at final parameter estimates  
lines(x,getPred(as.list(coef(nls.out)), x), col=2, lwd=2)

## summary information on parameter estimates
summary(nls.out) 

###### example 2 

## function to simulate data 
f <- function(TT, tau, N0, a, f0) {
    expr <- expression(N0*exp(-TT/tau)*(1 + a*cos(f0*TT)))
    eval(expr)
}

## helper function for an analytical gradient 
j <- function(TT, tau, N0, a, f0) {
    expr <- expression(N0*exp(-TT/tau)*(1 + a*cos(f0*TT)))
    c(eval(D(expr, "tau")), eval(D(expr, "N0" )),
      eval(D(expr, "a"  )), eval(D(expr, "f0" )))
}

## values over which to simulate data 
TT <- seq(0, 8, length=501)

## parameter values underlying simulated data  
p <- c(tau = 2.2, N0 = 1000, a = 0.25, f0 = 8)

## get data 
Ndet <- do.call("f", c(list(TT = TT), as.list(p)))
## with noise
N <- Ndet +  rnorm(length(Ndet), mean=Ndet, sd=.01*max(Ndet))

## plot the data to fit
par(mfrow=c(2,1), mar = c(3,5,2,1))  
plot(TT, N, bg = "black", cex = 0.5, main="data")

## define a residual function 
fcn     <- function(p, TT, N, fcall, jcall)
    (N - do.call("fcall", c(list(TT = TT), as.list(p))))

## define analytical expression for the gradient 
fcn.jac <- function(p, TT, N, fcall, jcall) 
    -do.call("jcall", c(list(TT = TT), as.list(p)))

## starting values 
guess <- c(tau = 2.2, N0 = 1500, a = 0.25, f0 = 10)

## to use an analytical expression for the gradient found in fcn.jac
## uncomment jac = fcn.jac
out <- nls.lm(par = guess, fn = fcn, jac = fcn.jac,
              fcall = f, jcall = j,
              TT = TT, N = N, control = nls.lm.control(nprint=1))

## get the fitted values 
N1 <- do.call("f", c(list(TT = TT), out$par))   

## add a blue line representing the fitting values to the plot of data 
lines(TT, N1, col="blue", lwd=2)

## add a plot of the log residual sum of squares as it is made to
## decrease each iteration; note that the RSS at the starting parameter
## values is also stored
plot(1:(out$niter+1), log(out$rsstrace), type="b",
main="log residual sum of squares vs. iteration number",
xlab="iteration", ylab="log residual sum of squares", pch=21,bg=2) 

## get information regarding standard errors
summary(out)