# Calculating and plotting confidence interval for Theil-Sen estimator

I'm using Wilcox's R functions (specifically, regplot) to plot a Theil-Sen estimator with a single predictor.

However, regplot doesn't plot a confidence band.

How do I calculate upper and lower percentile bootstrap confidence limits for each y value so that I can plot these limits?

The following R code is copied verbatim from Appendix C, page C-20 of U.S. EPA's Statistical Analysis of Groundwater Monitoring Data at RCRA Facilities; Unified Guidance; March 2009 (EPA 530/R-09-007; https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P10055GQ.TXT). The code was written by Kirk M. Cameron, Ph.D. of MacStat Consulting, Ltd.

# R script for Theil-Sen Confidence band
# Compute bootstrapped confidence band around Theil-Sen trend line
# user inputs: list of x-values, list of y-values, desired confidence level
# Note: replace numbers in parentheses below with specific x and y values
# corresponding to data-specific ordered pairs
# x-values should be numeric values representing sampling dates or events
# y-values should be concentration values corresponding to these dates or events
# Script produces a plot of the Theil-Sen trend line, the confidence band around the trend,
# and an overlay of the actual data values
x= c(89.6,90.1,90.8,91.1,92.1,93.1,94.1,95.6,96.1,96.3)
y= c(56,53,51,55,52,60,62,59,61,63)
conf = .90
elimna= function(m){
#
# remove any rows of data having missing values
m= as.matrix(m)
ikeep= c(1:nrow(m))
for(i in 1:nrow(m)) if (sum(is.na(m[i,])>=1)) ikeep[i]= 0
elimna= m[ikeep[ikeep>=1],]
elimna
}
theilsen2= function(x,y){
#
# Compute the Theil-Sen regression estimator
# Do not compute residuals in this version
# Assumes missing pairs already removed
#
ord= order(x)
xs= x[ord]
ys= y[ord]
vec1= outer(ys,ys,"-")
vec2= outer(xs,xs,"-")
v1= vec1[vec2>0]
v2= vec2[vec2>0]
slope= median(v1/v2)
coef= 0
coef[1]= median(y)-slope*median(x)
coef[2]= slope
list(coef=coef)
}
nb= 1000
temp= matrix(c(x,y),ncol=2)
temp= elimna(temp) #remove any pairs with missing values
x= temp[,1]
y= temp[,2]
n= length(x)
ord= order(x)
cut= min(x) + (0:100)*(max(x)-min(x))/100 #compute 101 cut pts
t0= theilsen2(x,y) #compute trend line on original data
tmp= matrix(nrow=nb,ncol=101)
for (i in 1:nb) {
idx= sample(ord,n,rep=T)
xboot= x[idx]
yboot= y[idx]
tboot= theilsen2(xboot,yboot)
tmp[i,]= tboot$$coef[1] + cut*tboot$$coef[2]
}
lb= 0; ub= 0
for (i in 1:101){
lb[i]= quantile(tmp[,i],c((1-conf)/2))
ub[i]= quantile(tmp[,i],c((1+conf)/2))
}
tband= list(xcut=cut,lo=lb,hi=ub,ths0=t0)
yt= tband$$ths0$$coef[1] + tband$$ths0$$coef[2]*tband$$xcut plot(yt~tband$$xcut,type='l',xlim=range(x),ylim=c(min(tband$$lo),max(tband$$hi)),xlab='Date',ylab='Conc')
points(x,y,pch=16)
lines(tband$$hi~tband$$xcut,type='l',lty=2)
lines(tband$$lo~tband$$xcut,type='l',lty=2)