Description Usage Arguments Details Value Note Author(s) See Also Examples
An utility function to perform a series of t-tests on several combinations of levels of one or more variables. P-value correction methods can be specified.
1 2 | tpairs(dat, vars, contr, dep, wid, p.adjust.methods = "none",
paired = FALSE, ...)
|
dat |
a |
vars |
a vector of characters indicating the names of the independent variables to be considered for the t-tests. If length vars > 1, a new variable will be created by combining the levels of the variables specified in |
contr |
a list specifying the t-test to be performed. If |
dep |
a string specifying the name of the dependent variable (a column in |
wid |
optional: a string specifying the variable that encodes the repeated measure identifier. |
p.adjust.methods |
p-value correction methods as in |
paired |
logical. If TRUE, paired t-tests are performed. |
... |
Arguments passed to |
The contr
argument allows to specify a subset of combinations of levels of the variables listed in vars
to be performed. The contrasts have to be specified as a list of lists. Each list within the main list contains two vectors with strings. The strings specify the levels of the variables in vars
that have to be compared. The convention is to specify the levels separated by an underscore. For example suppose that vars=c("height", "color")
, and
contr=list(list(c("high_red"), c("low_red")), list(c("high_green", "low_green")))
will perform two t-tests. The first t-test it will compare the mean for height=="high"
and color=="red"
with the mean of height=="low"
, and color=="red"
. The second t-test will compare the mean of height=="high"
, and color=="green"
with the mean of height=="low"
and color=="green"
. If a p-value correction method is specified, only p-values of the t-test carried out will be taken into account.
A data.frame
with the results of the t-tests specified.
The means for the combinations of the variables taken into account are computed internally by tpairs
, according to the specification in vars
, dep
, wid
.
The function is not able to deal with mixed within and between t-tests. Please, perform the two kinds of t-tests separately and take into consideration if the variables specified in vars
are mixed (both within and between).
Giorgio Arcara
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | # simulate some subjects
subjRT=rnorm(20, 500, 100)
#simulate the effects of three experimental conditions for each subject
condA=rnorm(20, 50, 10)
condB=rnorm(20, -40, 10)
condC=rnorm(20, 20, 10)
#create a data frame
dat=data.frame(Subject=rep(1:20,3),
condition=c(rep("A", 20), rep("B", 20), rep("C", 20)),
RT=c(subjRT+condA, subjRT+condB, subjRT+condC ))
#perform pairwise t.test
tpairs(dat, "condition", "all", "RT", "Subject", var.equal=TRUE)
|
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