# ppoints

##### Ordinates for Probability Plotting

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)`

.

- Keywords
- distribution, arith, dplot

##### Usage

`ppoints(n, a = ifelse(n <= 1="" 3="" 10,="" 8,="" 2))<="" div="">`

##### 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

##### Examples

`library(stats)`

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

*Documentation reproduced from package stats, version 3.2.5, License: Part of R 3.2.5*