ppoints                package:stats                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 generated 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.

     The choice of 'a' follows the documentation of the function of the
     same name in Becker _et al_ (1988), and appears to have been
     motivated by results from Blom (1958) on approximations to expect
     normal order statistics (see also 'quantile').

References:

     Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) _The New S
     Language_. Wadsworth & Brooks/Cole.

     Blom, G. (1958) _Statistical Estimates and Transformed Beta
     Variables._ Wiley

See Also:

     'qqplot', 'qqnorm'.

Examples:

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

