Ordinates for Probability Plotting
Generates the sequence of probability points
(1:m - a)/(m + (1-a)-a)
m is either
ppoints(n, a = ifelse(n <= 1="" 3="" 10,="" 8,="" 2))<="" div="">=>
- either the number of points generated or a vector of observations.
- the offset fraction to be used; typically in $(0,1)$.
If $0 < a < 1$, the resulting values are within $(0,1)$
In any case, the resulting sequence is symmetric in $[0,1]$, i.e.,
p + rev(p) == 1.
ppoints() is used in
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
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
ppoints(4) # the same as ppoints(1:4) ppoints(10) ppoints(10, a = 1/2)