Initial Remark: Reload this page if formulas don’t display well!

As promised, here is the second part on how to obtain confidence intervals for fitted values obtained from nonlinear regression via *nls* or *nlsLM* (package ‘minpack.lm’).

I covered a Monte Carlo approach in http://rmazing.wordpress.com/2013/08/14/predictnls-part-1-monte-carlo-simulation-confidence-intervals-for-nls-models/, but here we will take a different approach: First- and second-order Taylor approximation around : .

Using Taylor approximation for calculating confidence intervals is a matter of propagating the uncertainties of the parameter estimates obtained from *vcov(model)* to the fitted value. When using first-order Taylor approximation, this is also known as the “Delta method”. Those familiar with error propagation will know the formula

.

**Heavily** underused is the matrix notation of the famous formula above, for which a good derivation can be found at http://www.nada.kth.se/~kai-a/papers/arrasTR-9801-R3.pdf:

,

where is the gradient vector of first-order partial derivatives and is the variance-covariance matrix. This formula corresponds to the first-order Taylor approximation. Now the problem with first-order approximations is that they assume linearity around . Using the “Delta method” for nonlinear confidence intervals in *R* has been discussed in http://thebiobucket.blogspot.de/2011/04/fit-sigmoid-curve-with-confidence.html or http://finzi.psych.upenn.edu/R/Rhelp02a/archive/42932.html.

For highly nonlinear functions we need (at least) a second-order polynomial around to realistically estimate the surrounding interval (red is linear approximation, blue is second-order polynomial on a sine function around ):

Interestingly, there are also matrix-like notations for the second-order mean and variance in the literature (see http://dml.cz/dmlcz/141418 or http://iopscience.iop.org/0026-1394/44/3/012/pdf/0026-1394_44_3_012.pdf):

Second-order mean: .

Second-order variance: ,

where is the Hessian matrix of second-order partial derivatives and is the matrix trace (sum of diagonals).

Enough theory, for wrapping this all up we need three utility functions:

1) *numGrad* for calculating numerical first-order partial derivatives.

numGrad <- function(expr, envir = .GlobalEnv) { f0 <- eval(expr, envir) vars <- all.vars(expr) p <- length(vars) x <- sapply(vars, function(a) get(a, envir)) eps <- 1e-04 d <- 0.1 r <- 4 v <- 2 zero.tol <- sqrt(.Machine$double.eps/7e-07) h0 <- abs(d * x) + eps * (abs(x) < zero.tol) D <- matrix(0, length(f0), p) Daprox <- matrix(0, length(f0), r) for (i in 1:p) { h <- h0 for (k in 1:r) { x1 <- x2 <- x x1 <- x1 + (i == (1:p)) * h f1 <- eval(expr, as.list(x1)) x2 <- x2 - (i == (1:p)) * h f2 <- eval(expr, envir = as.list(x2)) Daprox[, k] <- (f1 - f2)/(2 * h[i]) h <- h/v } for (m in 1:(r - 1)) for (k in 1:(r - m)) { Daprox[, k] <- (Daprox[, k + 1] * (4^m) - Daprox[, k])/(4^m - 1) } D[, i] <- Daprox[, 1] } return(D) }

2) *numHess* for calculating numerical second-order partial derivatives.

numHess <- function(expr, envir = .GlobalEnv) { f0 <- eval(expr, envir) vars <- all.vars(expr) p <- length(vars) x <- sapply(vars, function(a) get(a, envir)) eps <- 1e-04 d <- 0.1 r <- 4 v <- 2 zero.tol <- sqrt(.Machine$double.eps/7e-07) h0 <- abs(d * x) + eps * (abs(x) < zero.tol) Daprox <- matrix(0, length(f0), r) Hdiag <- matrix(0, length(f0), p) Haprox <- matrix(0, length(f0), r) H <- matrix(NA, p, p) for (i in 1:p) { h <- h0 for (k in 1:r) { x1 <- x2 <- x x1 <- x1 + (i == (1:p)) * h f1 <- eval(expr, as.list(x1)) x2 <- x2 - (i == (1:p)) * h f2 <- eval(expr, envir = as.list(x2)) Haprox[, k] <- (f1 - 2 * f0 + f2)/h[i]^2 h <- h/v } for (m in 1:(r - 1)) for (k in 1:(r - m)) { Haprox[, k] <- (Haprox[, k + 1] * (4^m) - Haprox[, k])/(4^m - 1) } Hdiag[, i] <- Haprox[, 1] } for (i in 1:p) { for (j in 1:i) { if (i == j) { H[i, j] <- Hdiag[, i] } else { h <- h0 for (k in 1:r) { x1 <- x2 <- x x1 <- x1 + (i == (1:p)) * h + (j == (1:p)) * h f1 <- eval(expr, as.list(x1)) x2 <- x2 - (i == (1:p)) * h - (j == (1:p)) * h f2 <- eval(expr, envir = as.list(x2)) Daprox[, k] <- (f1 - 2 * f0 + f2 - Hdiag[, i] * h[i]^2 - Hdiag[, j] * h[j]^2)/(2 * h[i] * h[j]) h <- h/v } for (m in 1:(r - 1)) for (k in 1:(r - m)) { Daprox[, k] <- (Daprox[, k + 1] * (4^m) - Daprox[, k])/(4^m - 1) } H[i, j] <- H[j, i] <- Daprox[, 1] } } } return(H) }

And a small function for the matrix trace:

tr <- function(mat) sum(diag(mat), na.rm = TRUE)

1) and 2) are modified versions of the genD function in the “numDeriv” package that can handle expressions.

Now we need the *predictNLS* function that wraps it all up:

predictNLS <- function( object, newdata, interval = c("none", "confidence", "prediction"), level = 0.95, ... ) { require(MASS, quietly = TRUE) interval <- match.arg(interval) ## get right-hand side of formula RHS <- as.list(object$call$formula)[[3]] EXPR <- as.expression(RHS) ## all variables in model VARS <- all.vars(EXPR) ## coefficients COEF <- coef(object) ## extract predictor variable predNAME <- setdiff(VARS, names(COEF)) ## take fitted values, if 'newdata' is missing if (missing(newdata)) { newdata <- eval(object$data)[predNAME] colnames(newdata) <- predNAME } ## check that 'newdata' has same name as predVAR if (names(newdata)[1] != predNAME) stop("newdata should have name '", predNAME, "'!") ## get parameter coefficients COEF <- coef(object) ## get variance-covariance matrix VCOV <- vcov(object) ## augment variance-covariance matrix for 'mvrnorm' ## by adding a column/row for 'error in x' NCOL <- ncol(VCOV) ADD1 <- c(rep(0, NCOL)) ADD1 <- matrix(ADD1, ncol = 1) colnames(ADD1) <- predNAME VCOV <- cbind(VCOV, ADD1) ADD2 <- c(rep(0, NCOL + 1)) ADD2 <- matrix(ADD2, nrow = 1) rownames(ADD2) <- predNAME VCOV <- rbind(VCOV, ADD2) NR <- nrow(newdata) respVEC <- numeric(NR) seVEC <- numeric(NR) varPLACE <- ncol(VCOV) outMAT <- NULL ## define counter function counter <- function (i) { if (i%%10 == 0) cat(i) else cat(".") if (i%%50 == 0) cat("\n") flush.console() } ## calculate residual variance r <- residuals(object) w <- weights(object) rss <- sum(if (is.null(w)) r^2 else r^2 * w) df <- df.residual(object) res.var <- rss/df ## iterate over all entries in 'newdata' as in usual 'predict.' functions for (i in 1:NR) { counter(i) ## get predictor values and optional errors predVAL <- newdata[i, 1] if (ncol(newdata) == 2) predERROR <- newdata[i, 2] else predERROR <- 0 names(predVAL) <- predNAME names(predERROR) <- predNAME ## create mean vector meanVAL <- c(COEF, predVAL) ## create augmented variance-covariance matrix ## by putting error^2 in lower-right position of VCOV newVCOV <- VCOV newVCOV[varPLACE, varPLACE] <- predERROR^2 SIGMA <- newVCOV ## first-order mean: eval(EXPR), first-order variance: G.S.t(G) MEAN1 <- try(eval(EXPR, envir = as.list(meanVAL)), silent = TRUE) if (inherits(MEAN1, "try-error")) stop("There was an error in evaluating the first-order mean!") GRAD <- try(numGrad(EXPR, as.list(meanVAL)), silent = TRUE) if (inherits(GRAD, "try-error")) stop("There was an error in creating the numeric gradient!") VAR1 <- GRAD %*% SIGMA %*% matrix(GRAD) ## second-order mean: firstMEAN + 0.5 * tr(H.S), ## second-order variance: firstVAR + 0.5 * tr(H.S.H.S) HESS <- try(numHess(EXPR, as.list(meanVAL)), silent = TRUE) if (inherits(HESS, "try-error")) stop("There was an error in creating the numeric Hessian!") valMEAN2 <- 0.5 * tr(HESS %*% SIGMA) valVAR2 <- 0.5 * tr(HESS %*% SIGMA %*% HESS %*% SIGMA) MEAN2 <- MEAN1 + valMEAN2 VAR2 <- VAR1 + valVAR2 ## confidence or prediction interval if (interval != "none") { tfrac <- abs(qt((1 - level)/2, df)) INTERVAL <- tfrac * switch(interval, confidence = sqrt(VAR2), prediction = sqrt(VAR2 + res.var)) LOWER <- MEAN2 - INTERVAL UPPER <- MEAN2 + INTERVAL names(LOWER) <- paste((1 - level)/2 * 100, "%", sep = "") names(UPPER) <- paste((1 - (1- level)/2) * 100, "%", sep = "") } else { LOWER <- NULL UPPER <- NULL } RES <- c(mu.1 = MEAN1, mu.2 = MEAN2, sd.1 = sqrt(VAR1), sd.2 = sqrt(VAR2), LOWER, UPPER) outMAT <- rbind(outMAT, RES) } cat("\n") rownames(outMAT) <- NULL return(outMAT) }

With all functions at hand, we can now got through the same example as used in the Monte Carlo post:

DNase1 <- subset(DNase, Run == 1)

fm1DNase1 <- nls(density ~ SSlogis(log(conc), Asym, xmid, scal), DNase1)

> predictNLS(fm1DNase1, newdata = data.frame(conc = 5), interval = "confidence")

.

mu.1 mu.2 sd.1 sd.2 2.5% 97.5%

[1,] 1.243631 1.243288 0.03620415 0.03620833 1.165064 1.321511

The errors/confidence intervals are larger than with the MC approch (who knows why?) but it is very interesting to see how close the second-order corrected mean (1.243288) comes to the mean of the simulated values from the Monte Carlo approach (1.243293)!

The two approach (MC/Taylor) will be found in the *predictNLS* function that will be part of the “propagate” package in a few days at CRAN…

Cheers,

Andrej

Hi Anrej,

This is really cool and I have been looking for something that can calculate a single confidence interval for NLS models for quite some time. I have tried to use your function on some of the models that I have and it seems to work perfectly for models with a single predictor variable. However, when I use multiple predictor variables it is not working. Any ideas how to work around that?

With kind regards,

Robbie

I think ‘nls’ is only applicable for one-dimensional problems… When you say “multiple predictors”, do you mean that you are i.e. fitting surfaces and use x/y as predictor variables?

Greets,

Andrej

Hi Andrej,

I have for example a model in which I have a variable that is modelled against 3 predictor variables:

model <- nls(rate ~ ((a * prey)* size^b)/((1 + c * prey) * (1 + d * (pred-1)))….)

In which a, b, c and d are the parameter estimates and prey, pred and size are the predictor variables. This seem to work perfectly and the outputs seem to make sense. Only those dreadful confidence intervals.

I am not a statistician so you could be right that NLS can only be used for models with one predictor variable. Nevertheless, I have seen it being used this way in one of the Use R! books: Nonlinear Regression with R by Ritz & Streibig (2008). They give an example somewhere on page 45.

I have been playing a bit with the function you give here to get it working for my model, but still without success (for now).

Thanks Robbie

@Robbie:

I will submit my new ‘propagate’ package to CRAN in the next days. This will have the predictNLS function included that does MC simulation and Taylor approximation under one hood. I will try to incorporate the handling of multiple predictor values, so stay tuned at these comments!

Cheers,

Andrej

Hi Andrej,

Looking forward to that, already have some research papers waiting here to implement this.

Thanks

[…] error to the fitted values of a nonlinear model of type nls or nlsLM. Please refer to my post here: http://rmazing.wordpress.com/2013/08/26/predictnls-part-2-taylor-approximation-confidence-intervals-…. * makeGrad, makeHess, numGrad, numHess are functions to create symbolical or numerical gradient […]

Andrej,

I am very pleased to see your predictNLS function made available in an R package. Thank you. Now, how about prediction intervals, as well as confidence and prediction bands (regions)? That would be super.

BTW, I notice the posts questioning the use of “nls” for models with multiple predictor variables. I have been regularly using “nls” to fit models with up to 5 predictors with great success. I hope your predictNLS performs appropriately in that context.

Best regards,

Leland

Hi Leland,

the last two examples in the ‘Examples’ section of ?predictNLS show you how to draw confidence/prediction intervals (by setting ‘interval’ in the function) and how to use multiple predictor values with error.

Drop me a note if it works in your hands!

Cheers,

Andrej