Hello, I have been using the QQ-Plot functions in R for a while now and I noticed what I believe is a major inconsistency in the way R estimates and plots the quantiles of a data set. To see this, let x be a vector of data (or just simulate some data) say 15 points. Then type qqnorm(x) By default, R will estimate the sample quantiles using qtype = 5 which is p[k] = (k-0.5)/n which is fine for now. Next, if we want to plot a line through the first and third quantiles we would type qqline(x, datax = FALSE, distribution = qnorm, probs = c(0.25,0.75)) which by default uses qtype = 7 when estimating the quantiles for the sample!!! To convince yourself it uses qtype = 7 just type the previous command line and use a , qtype = 7 as follows qqline(x, datax = FALSE, distribution = qnorm, probs = c(0.25,0.75), qtype = 7) which is the same line that was plotted before. The fact that it estimates the sample quantiles using 2 different methods and plots them on the same graph doesn't make any sense to me. For example, if I wanted to draw a line through the first and last quantiles in my sample using a particular qtype say qtype = 6, I would calculate the corresponding probabilities using qytpe = 6 which is p[k] = k/(n+1) and enter the following command qqline(x, datax = FALSE, distribution = qnorm, probs = c(1/16,15/16), qtype = 6) which doesn't pass through the first and last quantiles since it was originally plotted using qtype = 5. To see that R plotted this using qtype = 5, write the above command using qtype = 5. This is p[k] = (k-0.5)/n and therefore the first and last quantiles are 0.5/15 and 14.5/15 respectively. The command is the following qqline(x, datax = FALSE, distribution = qnorm, probs = c(0.5/15,14.5/15), qtype = 5) and this line should run directly through the first and last sample quantiles as originally desired. Why doesn't R re-plot the data using the specified qtype when drawing the qqline? or Why would you want to view a plot that used different methods for estimating quantiles? I completely understand why we would want to use different quantile estimators, especially if we want an unbiased (or appoximately) estimator in some sense (qype 8 and 9). However, using 2 different estimators on the same plot could be dangerous. [[alternative HTML version deleted]]