ppoints {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)

[Package

*stats* version 2.5.0

Index]