Pairwise t tests

Description

Calculate pairwise comparisons between group levels with corrections for multiple testing

Usage

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pairwise.t.test(x, g, p.adjust.method = p.adjust.methods,
                pool.sd = !paired, paired = FALSE,
                alternative = c("two.sided", "less", "greater"),
                ...)

Arguments

x

response vector.

g

grouping vector or factor.

p.adjust.method

Method for adjusting p values (see p.adjust).

pool.sd

switch to allow/disallow the use of a pooled SD

paired

a logical indicating whether you want paired t-tests.

alternative

a character string specifying the alternative hypothesis, must be one of "two.sided" (default), "greater" or "less". Can be abbreviated.

...

additional arguments to pass to t.test.

Details

The pool.sd switch calculates a common SD for all groups and uses that for all comparisons (this can be useful if some groups are small). This method does not actually call t.test, so extra arguments are ignored. Pooling does not generalize to paired tests so pool.sd and paired cannot both be TRUE.

Only the lower triangle of the matrix of possible comparisons is being calculated, so setting alternative to anything other than "two.sided" requires that the levels of g are ordered sensibly.

Value

Object of class "pairwise.htest"

See Also

t.test, p.adjust

Examples

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attach(airquality)
Month <- factor(Month, labels = month.abb[5:9])
pairwise.t.test(Ozone, Month)
pairwise.t.test(Ozone, Month, p.adj = "bonf")
pairwise.t.test(Ozone, Month, pool.sd = FALSE)
detach()

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