\name{combine.p}
\alias{combine.p}
\title{Combine probabilities from tests of significance}
\usage{combine.p(x, y = NULL, method = c("fisher", "winer", "stouffer"))}
\arguments{
    \item{x}{a vector of p.values}
    \item{y}{a vector of degrees of freedom for Winer method}
    \item{method}{a character string indicating which method to use.
      One of "fisher", "winer", or "stouffer".  No abbreviations
      accepted, defaults to "fisher".}
}

\description{
    \code{combine.p} combines probabilities from multiple significance
    tests using one of Fisher, Winer or Stouffer methods.
}

\details{
  Uses one of three methods to combine probabilities from multiple
  significance tests.

  if \code{method} is \code{"fisher"}, the probabilities given in
  \code{x} are tested against the null hypothesis that they are chi
  square distributed with 2k degrees of freedom, where k is the number
  of tests.

  if \code{method} is \code{"winer"}, the probabilities given in
  \code{x} are converted to t-scores, summed and divided by the square
  root of the degrees of freedom for each t after each df has been
  divided by df-2.  See Wolf (1986, pp 20) for rationale.

  if \code{method} is \code{"stouffer"}, the probabilities given in
  \code{x} are converted to z-scores, summed and divided by the square
  root of the number of tests.  See Wolf (1986, pp 20) for rationale.

}

\value{
    A list with class \code{"htest"} containing the following
    components:
    \item{statistic}{the value the chi square test statistic.}
    \item{parameter}{the degrees of freedom.}
    \item{p.value}{the p-value for the test.}
    \item{method}{a character string indicating the method.}
}

\references{
  Ronald A. Fisher (1958),
  \emph{Statistical Methods for Research Workers}, 13th ed.
  New York: Hafner Publishing.
  Section 21.1
  Pages 99--101.

  Robert R. Sokal & F. James Rohlf (1995),
  \emph{Biometry}, 3rd ed.
  New York: W. H. Freeman & Company.
  Section 18.1.
  Pages 794--797.

  Fredric M. Wolf (1986),
  \emph{Meta-analysis: quantitative methods for research synthesis}
  Quantitative Applications in the Social Sciences, vol 59
  Newbury Park: Sage Publications
  Pages 18--23.
}
\examples{
## Example from Fisher (1958) p 100.
> combine.p(c(.145,.263,.087))

	Fisher's combined probability

data:  
Chi Square = 11.4169, X-squared df = 6, p-value = 0.07631


## Example from Sokal & Rohlf p 795)
> combine.p(c(.073,.086,.10,.08,.06))

	Fisher's combined probability

data:  
Chi Square = 25.4249, X-squared df = 10, p-value = 0.004596

## Example from Wolf (1986) p 22
> combine.p(c(.004,.97,.024,.06),method="stouffer")

	Stouffer's combined probability

data:  
Z score = 2.1517, No df = NA, p-value = 0.01571

## Example from Wolf (1986) p 22
> combine.p(c(.004,.97,.024,.06),c(80,60,200,20),method="winer")

	Winer's combined probability

data:  
Z score = 2.1601, No df = NA, p-value = 0.01538

> 
}
\keyword{htest}