# How to calculate confidence intervals for power function ($y=ax^b$) in R using NLS

I hope you can help me get some confidence in my confidence interval... I am trying to get the confidence interval for a particular (threshold) point on a predicted curve.

I find the confidence interval (C.I.) looks funny, so I would like to know whether what I am doing is correct... Thank you for any advice you can give me!

#Here is the data to which I fit a nonlinear least squares regression (power function)
y <-c(0.5745373, 0.8255836, 0.7139635, 0.6318004, 0.8688738, 0.8341626, 0.9278573, 0.6548638,
0.6995722, 0.8410211, 1.0000000, 0.8512973, 0.7917790, 0.5722589, 0.6918069, 0.7117084,
0.5279689, 0.6121315, 0.5869292, 0.7332605, 0.5991816, 0.5470566, 0.5987166, 0.4440854,
0.3719892, 0.4394820, 0.4410862, 0.4966288, 0.4291826, 0.4221613, 0.3951223, 0.3595973,
0.4617549, 0.5106482, 0.5939970, 0.5340835, 0.6036920, 0.5181577, 0.3892170, 0.3667581)

x <- c(0.95149254, 0.57954545, 0.56763699, 0.57089552, 1.00000000, 0.66870629, 0.70833333,
0.53125000, 0.58776596, 0.78061224, 0.61551724, 0.63750000, 0.85000000, 0.52397260,
0.66870629, 0.50328947, 0.52192982, 0.40691489, 0.36016949, 0.37500000, 0.35915493,
0.53968254, 0.36334197, 0.23486842, 0.19615385, 0.35961538, 0.30039267, 0.26424870,
0.54838710, 0.23363874, 0.26936620, 0.09514925, 0.15324519, 0.32465278, 0.33909574,
0.31587838, 0.24401914, 0.28813559, 0.23181818, 0.29272959)

plot(x,y, xlim=c(0,1), ylim=c(0,1))
m1 <- nls(y~a*x^b, start=list(a=2,b=0.2))
summary(m1)
points(0.2276312,0.4595148, col='red', pch=17) # this is the threshold value I
# previously calculated


now I would like to know the Confidence Interval of the threshold

#lower CI
0.86888-(2*0.04578) # intercept minus 2* S.E.
0.43042-(2*0.06333) # exponent minus 2*S.E.
curve((0.77732*x^0.30376), add=TRUE) # plot the lower C.I.
0.77732*0.2276312^0.30376 # Now I enter the x coordinate of threshold point in the
# Confidence interval function, to get the y coordinate
points(0.2276312,0.4958526, col='red', pch="-", cex=1.5) # plotting the CI of threshold value

#Upper CI
0.86888+(2*0.04578) # intercept plus 2* S.E.
0.43042+(2*0.06333) # exponent plus 2*S.E.
curve((0.96044*x^0.55708), add=TRUE) # plot upper C.I.
0.96044*0.2276312^0.55708 # Now I enter the x coordinate of threshold point in the
# Confidence interval function, to get the y coordinate
points(0.2276312,0.4211113, col='red', pch="-", cex=1.5) # plot the CI of threshold value
lines(c(0.2276312,0.2276312),c(0.4211113,0.4958526), col="red")

# then check whether correct
confint(m1) # more or less, but slightly different, don't understand why?


To me, a C.I. that looks like this would make more sense...

curve((0.96044*x^0.30376), col="green",add=TRUE)


but that would be mixing the intercept of upper and exponent of lower C.I. and vice versa

My question is then, are the black Confidence Intervals correct (and thus the thresholds red confidence interval?

• – Roland Jun 29 '13 at 12:09
• The confint function is from the MASS package. Section 8.4 of Modern Applied Statistics with S (the manual for MASS) discusses why you are getting different answers than confint. – Tom Jun 29 '13 at 14:05
• Welcome to the site, @EHR. Please don't cross-post. You need to decide if you think of this question as primarily an R programming question (how to use R to do this) or a conceptual statistical question (which is the right CI & why), and then delete the other question. – gung - Reinstate Monica Jun 29 '13 at 15:34
• @ Tom, thank you for the tip. For anyone else reading this, here is the text stat.ubc.ca/~d.lee/TA/2011/Texts/MASS.pdf – EHR Jun 29 '13 at 15:38
• $dy/dx$ refers to the slope of the curve. Calculus tells us the slope of $y=ax^b$ equals $(ba)x^{b-1}$. Obtain your threshold like this: f <- function(x) coeff[1]*x ^ coeff[2]; fp <- function(x) coeff[1] * coeff[2] * x^(coeff[2]-1); secant <- (f(max(x)) - f(min(x))) / (max(x) - min(x)); threshold <- uniroot(function(x) fp(x) - secant, c(0,1))$root. It equals$0.4219369.$How are the$x$values in the data chosen? I don't believe the maximum value of$x=1$is random, but the minimum value of$x=.095$looks like a measurement. To what extent could these extreme values vary among experiments? – whuber Jul 2 '13 at 17:49 ## 1 Answer ### Analysis According to comments to the question, the "threshold" is determined this way: I calculate where the derivative of the curve equals the derivative of the secant, which is the straight line formed by joining the lowest and highest points on the curve. The "lowest" and "highest" points are at the extremes of the$x$values and are derived from the data. ### Proposed solution Because the secant is determined by the extremes of$x$, how they are determined is critical. I therefore suggest estimating the bias in the estimation of the threshold,$t$, as well as a confidence interval for it, using a semi-parametric bootstrap. In this method the model is: • The x values,$x_i$, are drawn independently and randomly from a distribution. (It looks like they tend to lie between$0$and$1$.) This is the "parametric" part of the bootstrap. We will need to explore how the results depend on assumptions about this distribution. • The y values,$y_i = a x_i^b$, are computed using the fitted parameters$\hat{a}$and$\hat{b}$from the original data in place of the unknown values$a$and$b$, respectively. • To these y values are added "errors" drawn independently and with replacement from the residuals of the original fit. This is the "nonparametric" part of the bootstrap: we have assumed that the vertical deviations of the original data from the correct curve are independent and identically distributed and we have taken the residuals from the original fit to be a representative sample of that distribution. • The model is fit to the$(x_i, y_i)$pairs and a threshold for them is computed as described previously. The distribution of these thresholds is the bootstrap distribution. Any deviation between its central location and our original estimate of the threshold,$\hat{t}$, gives us an additive bias correction factor. The spread of this distribution lets us erect approximate confidence intervals around the (bias-corrected) estimate. The bootstrap distribution is estimated by performing a large (but necessarily finite) number of simulations from the model. Usually a thousand or so iterations are enough to give good information. ### Example scenarios Here are results for three distributions of$X$: the original one (in which the$x_i$never vary), a uniform distribution, and a Beta distribution that approximates the distribution of the original data.$10^4$iterations were used. Each plot marks the estimated threshold$\hat{t}$with a vertical red line. Because that line is centered on the "Original X" histogram, we deduce there is little bias and can take the extreme values of this histogram as approximate confidence limits for$t$: roughly from$0.412$to$0.433$(95% confidence). The second and third plots show how exquisitely sensitive$\hat{t}$is to the distribution of the$x_i$and how biased an estimator it might be: notice how much wider the range of threshold is on their horizontal axes. In the second plot, where it is likely the smallest$x_i$is close to$0$and the largest$x_i$might be substantially less than$1$, the reference slope ("secant") is relatively high, pushing the estimated threshold to the left where the curve has greater slopes. The bias is large, approximately$+0.12$, merely because the largest$x_i$in the dataset is$1$and the smallest,$0.095$, is fairly far from zero. In the third plot the bias has disappeared (!) but the range of estimated thresholds is still wide. A symmetric 95% confidence interval for$t$is$[0.33, 0.52]$(based on$10^4$iterations). I suspect the lack of bias may be a bit of luck because the$x_i$are centered fairly close to the original$\hat{t}$. This can be confirmed by drawing the$x_i$from other distributions, such as a Beta(4,1) distribution (centered near$2/3$): the bias is now$-0.21$. This is about as far as we can reasonably take the analysis without knowing more about how the$x_i$are determined. The results are interesting insofar as they reveal how important it is to understand how the data arise: there is a huge range of answers available here, all of which are consistent with the numbers, but only some of which could be appropriate for what actually occurred during this study. ### Code R code to produce these results follows. (Change n.iter to obtain other than$10^3$iterations.) y <-c(0.5745373, 0.8255836, 0.7139635, 0.6318004, 0.8688738, 0.8341626, 0.9278573, 0.6548638, 0.6995722, 0.8410211, 1.0000000, 0.8512973, 0.7917790, 0.5722589, 0.6918069, 0.7117084, 0.5279689, 0.6121315, 0.5869292, 0.7332605, 0.5991816, 0.5470566, 0.5987166, 0.4440854, 0.3719892, 0.4394820, 0.4410862, 0.4966288, 0.4291826, 0.4221613, 0.3951223, 0.3595973, 0.4617549, 0.5106482, 0.5939970, 0.5340835, 0.6036920, 0.5181577, 0.3892170, 0.3667581) x <- c(0.95149254, 0.57954545, 0.56763699, 0.57089552, 1.00000000, 0.66870629, 0.70833333, 0.53125000, 0.58776596, 0.78061224, 0.61551724, 0.63750000, 0.85000000, 0.52397260, 0.66870629, 0.50328947, 0.52192982, 0.40691489, 0.36016949, 0.37500000, 0.35915493, 0.53968254, 0.36334197, 0.23486842, 0.19615385, 0.35961538, 0.30039267, 0.26424870, 0.54838710, 0.23363874, 0.26936620, 0.09514925, 0.15324519, 0.32465278, 0.33909574, 0.31587838, 0.24401914, 0.28813559, 0.23181818, 0.29272959) # # Fit a curve y = a*x^b to the data using nonlinear least squares, # compute the "threshold," and return its value. # (Called by sim.) # fit <- function(x, y, a=1, b=0.5) { m <- nls(y ~ a * x^b, start=list(a=a, b=b)) coeff <- m$m$getPars() f <- function(x) coeff[1] * x^coeff[2] fp <- function(x) coeff[1] * coeff[2] * x^(coeff[2]-1) x.min <- min(x) x.max <- max(x) secant <- (f(x.max) - f(x.min)) / (x.max - x.min); threshold <- uniroot(function(x) fp(x) - secant, c(0,10^6))$root #$return(threshold) } # # Perform a bootstrap simulation with the specified model: # n is the number of iterations # f is the original estimated curve (as a vectorized function) # e is the array of residuals to the initial fit (it defines an empirical # distribution of residuals) # alpha and beta are parameters for a Beta distribution of the x[i] # x is the array of x[i] to use when not simulating the x[i]. # sim <- function(n = 1000, f, e, alpha=1, beta=1, x=NULL) { replicate(n, { if (is.null(x)) { x.0 <- rbeta(length(e), alpha, beta) } else { x.0 <- x } y.0 <- f(x.0) + sample(e, length(e), replace=TRUE) fit(x.0, y.0) }) } # # Obtain the initial fit. # m1 <- nls(y ~ a * x^b, start=list(a=1, b=0.5)) # # Compute the residuals e and the curve function f. # e <- residuals(m1) coeff <- m1$m\$getPars()
f <- function(x) coeff[1] * x^coeff[2]
#
# Prepare to bootstrap and plot several scenarios.
# Each scenario should take 2-3 seconds for n.iter=1000.
#
par(mfrow=c(1,3))
n.iter <- 10^3
b <- seq(0.15, 0.65, 0.025)
set.seed(17)
#
# Scenario 1: keep the x values fixed.
#
threshold <- sim(n.iter, f, e, x=x)
hist(threshold, main="Original X", freq=FALSE)
abline(v = fit(x, y), col="Red")
quantile(threshold, c(0.025, 0.975)) # A symmetric CI
#
# Scenario 2: vary the x values uniformly in [0,1].
#
threshold <- sim(n.iter, f, e, alpha=1, beta=1)
hist(threshold, main="X ~ Uniform(0,1)", freq=FALSE, breaks=b)
abline(v = fit(x, y), col="Red")
quantile(threshold, c(0.025,0.5, 0.975))
#
# Scenario 3: vary the x values according to Beta(4,4).
#
threshold <- sim(n.iter, f, e, alpha=4, beta=4)
hist(threshold, main="X ~ Beta(4,4)", freq=FALSE, breaks=b)
abline(v = fit(x, y), col="Red")
quantile(threshold, c(0.025, 0.975))

• Thank you @whuber, for taking the time to explain all this! I had started with the bootstrapping, but am a real beginner bootstrapper... Still do not fully understand though how you get the threshold from the secant line. I think our secant lines differ?. Would you be able to plot the curve and secant line with the bootstrapped CI's? Then we can compare. I would do it if I knew how to use/plot the CI's you produced... – EHR Jul 4 '13 at 7:25
• This is also interesting! quantitativeconservationbiology.wordpress.com/2013/07/02/… – EHR Jul 4 '13 at 12:23
• The threshold is computed by fit by finding the x value at which the slope of the fitted curve (calculated by fp) equals the slope of the secant (stored in secant). I can't compare this to your method because you haven't indicated how you "previously calculated" the threshold for your data and your fit. – whuber Jul 4 '13 at 14:37
• Ok, but then it should be the same, because that is also what I do when calculating the threshold (see comment above). To calculate the threshold, I calculate where the derivative of the curve equals the derivative of the secant. The curve is 0.86888*x^0.43042. Therefore I calculated when 0.37398*x^-0.56958 (derivative of curve) equals 0.86888 (derivative of secant). Then, x^-0.56958=0.86888/0.37398, so x^-0.56958=2.32333, making x=2.32333^(1/-0.56958) = 0.2276. Not sure then why it is not the same as your result... – EHR Jul 5 '13 at 6:27
• You are not computing the secant in the way you stated in your comments! You wrote it is "the straight line formed by joining the lowest and highest points on the curve." In response to a question about the domain of "the curve," you wrote "the endpoints are derived from the data." Well, your data go from min(x), which equals 0.095, to max(x), equal to 1. Those points are (0.095, 0.316) and (1, 0.867). The slope of the secant therefore equals (0.867 - 0.316) / (1 - 0.095) = 0.61. If you meant something else, just change the calculation of secant in the fit function. – whuber Jul 5 '13 at 13:01