ppoints(n, a = ifelse(n <= 10, 3/8, 1/2))
(1:m - a)/(m + (1-a)-a)
where m is either n, if length(n)==1, or
length(n).p + rev(p) == 1.
ppoints() is used in qqplot and qqnorm to generate
the set of probabilities at which to evaluate the inverse distribution.
qqplot, qqnorm.ppoints(4) # the same as ppoints(1:4) ppoints(10) ppoints(10, a=1/2)