two-tailed exact binomail test
Peter Dalgaard wrote:
katrina smith <chezsmithy at yahoo.co.uk> writes:
I am trying to find a definition for the two-tailed exact binomial test but have been unsuccessful. Can you help?
Just read binom.test. The relevant bit is this:
(m is the mean == n*p)
else if (x < m) {
i <- seq(from = ceiling(m), to = n)
y <- sum(dbinom(i, n, p) <= d * relErr)
pbinom(x, n, p) + pbinom(n - y, n, p, lower = FALSE)
}
i.e. we take the lower tail, including the value observed + the part
of the upper tail where the binomial density is less than or equal to
that of x (with a little fuzz added in). Symmetrically for observations
in the upper tail of course.
If you were looking for an "official" definition of the two sided
exact test, I don't think one exists. R's version is equivalent to the
likelihood ratio test, but there are alternatives (tail-balancing,
doubling the one-sided p, and maybe more).
there is a reference:
Section 2.4.2 ("Zweiseitige Tests in einparametrigen
Exponentialfamilien" - two sided tests in one-parameter exponential
families) in
H. Witting (1985): Mathematische Statistik I. Teubner. Stuttgart
confer Satz 2.70, Korollar 2.73 (in case of symmetry)
and Beispiel 2.74 (application of Korollar 2.73 to binomial model for p=
0.5).
Matthias