R: Ordinates for Probability Plotting

ppoints {base}

R Documentation

Ordinates for Probability Plotting

Description

Generates the sequence of “probability” points (1:m - a)/(m + (1-a)-a) where m is either n, if length(n)==1, or length(n).

Usage

ppoints(n, a = ifelse(n <= 10, 3/8, 1/2))

Arguments

n

either the number of points generate or a vector of observations.

a

the offset fraction to be used; typically in (0,1).

Details

If 0 < a < 1, the resulting values are within (0,1) (excluding boundaries). In any case, the resulting sequence is symmetric in [0,1], i.e., p + rev(p) == 1.

ppoints() is used in qqplot and qqnorm to generate the set of probabilities at which to evaluate the inverse distribution.

Examples

ppoints(4) # the same as  ppoints(1:4)
ppoints(10)
ppoints(10, a=1/2)