\Header{ppoints}{Ordinates for Probability Plotting}
\keyword{dplot}{ppoints}
\keyword{arith}{ppoints}
\keyword{distribution}{ppoints}
\begin{Usage}
\begin{verbatim}
ppoints(n, a = ifelse(n <= 10, 3/8, 1/2))
\end{verbatim}
\end{Usage}
\begin{Description}
Generates the sequence of ``probability'' points
\texttt{(1:m - a)/(m + (1-a)-a)}
where \texttt{m} is either \texttt{n}, if \texttt{length(n)==1}, or
\texttt{length(n)}.\end{Description}
\begin{Details}
If \eqn{0 < a < 1}{}, the resulting values are within \eqn{(0,1)}{}
(excluding boundaries).
In any case, the resulting sequence is symmetric in \eqn{[0,1]}{}, i.e.,
\texttt{p + rev(p) == 1}.

\texttt{ppoints()} is used in \texttt{qqplot} and \texttt{qqnorm} to generate
the set of probabilities at which to evaluate the inverse distribution.\end{Details}
\begin{SeeAlso}
\texttt{\Link{qqplot}}, \texttt{\Link{qqnorm}}.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
ppoints(4) # the same as  ppoints(1:4)
ppoints(10)
ppoints(10, a=1/2)
\end{ExampleCode}
\end{Examples}

