# data from Table 1.2.1

tab121 <- c(7,  11,  15,  16,  20,  22,  24,  25,  29,  33,  
 34,  37,  41,  42,  49,  57,  66,  71,  84,  90) 


 # a function to calculate confidence intervals for percentiles
 # using the normal approximation to the binomial distribution
 # Note that both p and alpha are numbers between 0 and 1

cipercent <- function(y,p,alpha)
      {
        n <- length(y)
        a <- n*p - qnorm(1 -alpha/2)*sqrt(n*p*(1-p))
        b <- 1 +n*p + qnorm(1 -alpha/2)*sqrt(n*p*(1-p))
        ar <- round(a) ;  br <- round(b)
        lower <- y[order(y)[ar]]
        upper <- y[order(y)[br]]
       # print(c(n,a,b,ar,br,lower,upper))
        c(lower,upper)
      }

 # this gives the same results as the example in the text

cipercent(tab121,.25,.10)