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The other day I saw a plot with some data with a clear jump in it. So I wondered if it were possible to determine the position of that jump. Data with a jump is easy enough made:
library(ggplot2)
n <- 100
mydata <- data.frame(x=runif(n))
mydata$y <- rnorm(n,0,1) +ifelse(mydata$x>.5,0,1)
qplot(x=mydata$x,y=mydata$y)

Now to find where that jump was. After some consideration, the most simple way seemed appropriate. Just take any possible point and examine the corrected sums of squares of the parts left and right of the break.
mydata <- mydata[order(mydata$x),] ss <- function(x) sum((x-mean(x))^2) mids <- (mydata$x[-1]+mydata$x[-nrow(mydata)])/2 sa <- sapply(mids,function(x) ss(mydata$y[mydata$xx])) qplot(x=mids,y=sa, ylab=’Sum of squares’,xlab=’Position of break’) That was very simple. But still too complex. I realized I was recreating a tree with only one node. There is a way to get that first node more quickly: library(tree) tree(y~x,data=mydata,) node), split, n, deviance, yval * denotes terminal node 1) root 100 109.700 0.76210 2) x < 0.515681 58 50.430 1.23700 4) x < 0.0865511 7 8.534 0.56330 * 5) x > 0.0865511 51 38.280 1.33000 10) x < 0.144596 7 8.355 1.72600 * 11) x > 0.144596 44 28.650 1.26700 22) x < 0.276795 10 3.767 0.71330 * 23) x > 0.276795 34 20.920 1.43000 46) x < 0.380858 16 13.150 1.68100 * 47) x > 0.380858 18 5.856 1.20600 * 3) x > 0.515681 42 28.100 0.10580 6) x < 0.853806 29 13.950 0.28370 * 7) x > 0.853806 13 11.190 -0.29090 14) x < 0.938267 6 5.031 -0.70380 * 15) x > 0.938267 7 4.255 0.06312 * So, the first break is at 0.51. That was easy. #### In JAGS I like JAGS far too much. So, I wanted to do that in JAGS. And it was simple too. Just make two means and (assume) a common standard deviation. The break (called limit when I programmed this, but break is an inconvenient name for a variable in R) has a beta prior. library(R2jags) model1 <- function() { for ( i in 1:N ) { category[i] <- (xx[i]>limit) + 1 yy[i] ~ dnorm( mu[category[i]] , tau ) } tau <- 1/pow(sigma,2) sigma ~ dunif(0,100) mu ~ dnorm(0,1E-6) mu ~ dnorm(0,1E-6) limit ~ dbeta(1,1) } datain <- list(yy=mydata$y,xx=mydata$x,N=nrow(mydata)) params <- c('mu','sigma','limit') inits <- function() { list(mu = rnorm(2,0,1), sigma = runif(1,0,1), limit=runif(1,0,1)) } jagsfit <- jags(datain, model=model1, inits=inits, parameters=params, progress.bar=”gui”,n.iter=10000) jagsfit Inference for Bugs model at “C:/Users/…/Temp/RtmpuosigK/model176025bc6712.txt”, fit using jags, 3 chains, each with 10000 iterations (first 5000 discarded), n.thin = 5 n.sims = 3000 iterations saved mu.vect sd.vect 2.5% 25% 50% 75% 97.5% Rhat n.eff limit 0.528 0.042 0.474 0.504 0.516 0.541 0.629 1.001 3000 mu 1.212 0.128 0.959 1.128 1.213 1.296 1.465 1.002 1400 mu 0.122 0.152 -0.183 0.024 0.121 0.226 0.417 1.001 3000 sigma 0.922 0.069 0.795 0.872 0.917 0.967 1.070 1.002 1100 deviance 265.762 3.751 260.131 262.937 265.553 267.918 274.100 1.001 3000 For each parameter, n.eff is a crude measure of effective sample size, and Rhat is the potential scale reduction factor (at convergence, Rhat=1). DIC info (using the rule, pD = var(deviance)/2) pD = 7.0 and DIC = 272.8 DIC is an estimate of expected predictive error (lower deviance is better). So, the posterior median for the break is 0.516 with a slightly higher mean of 0.52. To make things more sweet, how about a nice combined plot? Extract the samples, take a hint from stackoverflow how to arrange the parts and a hint from google groups ggplot2 on the removal of the x=axis label. Mission accomplished. limit = as.numeric(jagsfit$BUGSoutput$sims.array[,,’limit’]) q1 <- qplot(x=mydata$x,y=mydata\$y,xlim=c(0,1),ylab='y') +
theme(axis.text.x = element_blank(),axis.title.x=element_blank(),
axis.ticks.x=element_blank())
d1 <- ggplot(,aes(x=limit)) + geom_density() +
scale_x_continuous(limits=c(0,1),’x’) +
scale_y_continuous(‘Density of break position’ )

library(gridExtra)
grid.arrange(q1,d1, nrow=2)