\name{cchart.u}
\alias{cchart.u}
%- Also NEED an '\alias' for EACH other topic documented here.
\title{ u-chart }
\description{
  This function builds a u-chart for the Poisson-based count data statistic.
}
\usage{
cchart.u(x1 = NULL, n1 = NULL, type = "norm", u1 = NULL, x2 = NULL, n2 = NULL, lambda = NULL, u2 = NULL)
}
%- maybe also 'usage' for other objects documented here.
\arguments{
  \item{x1}{ The phase I data that will be plotted (if it is a phase I chart). }
  \item{n1}{ A value or a vector of values specifying the sample sizes associated with each group for the phase I data. }
  \item{type}{ The type of u-chart to be plotted. The options are "norm" (traditional Shewhart u-chart), "CF" (improved u-chart) and "std" (standardized u-chart). If not specified, a Shewhart u-chart will be plotted. }
  \item{u1}{ The sample ratios used to estimate the Poisson parameter (lambda). (x1 / n1). }
  \item{x2}{ The phase II data that will be plotted in a phase II chart. }
  \item{n2}{ A value or a vector of values specifying the sample sizes associated with each group for the phase II data. }
  \item{lambda}{ The estimate of lambda. }
  \item{u2}{ The sample ratios of the phase II data (x2 / n2). }
}
\details{
  For a phase I u-chart, n1 must be specified and either x1 or u1.
  For a phase II u-chart, n2 must be specified, plus x2 or u2 and either phat, x1 and n1, or u1 and n1.
  It is important to note that the normal approximation used in the Shewhart u-chart is valid only for n*u large. For small n*p , it should be used an "improved u chart" (with non-normal correction) given by using the argument "CF".
}
\value{
  Returns a u-chart.
}
\author{ Daniela R. Recchia, Emanuel P. Barbosa }
\examples{
data(moonroof)
attach(moonroof)
cchart.u(x1 = yi[1:17], n1 = ni[1:17])
cchart.u(x1 = yi[1:17], n1 = ni[1:17], type = "CF", x2 = yi[18:34], n2 = ni[18:34])
cchart.u(type = "std", u2 = ui[18:34], n2 = ni[18:34], lambda = 1.4)
}