MeanDiffCI: Confidence Interval For Difference of Means

Description

Calculates the confidence interval for the difference of two means either the classical way or with the bootstrap approach.

Usage

MeanDiffCI(x, ...)
# S3 method for default
MeanDiffCI(x, y, method = c("classic", "norm", "basic", "stud", "perc", "bca"),
           conf.level = 0.95, sides = c("two.sided", "left", "right"), paired = FALSE,
           na.rm = FALSE, R = 999, ...)
# S3 method for formula
MeanDiffCI(formula, data, subset, na.action, ...)

Value

a numeric vector with 3 elements:

meandiff: the difference: mean(x) - mean(y)
lwr.ci: lower bound of the confidence interval
upr.ci: upper bound of the confidence interval

Arguments

x: a (non-empty) numeric vector of data values.
y: a (non-empty) numeric vector of data values.
method: a vector of character strings representing the type of intervals required. The value should be any subset of the values "classic", "norm", "basic", "stud", "perc", "bca". See boot.ci.
conf.level: confidence level of the interval.
sides: a character string specifying the side of the confidence interval, must be one of "two.sided" (default), "left" or "right". You can specify just the initial letter. "left" would be analogue to a hypothesis of "greater" in a t.test.
paired: a logical indicating whether you want confidence intervals for a paired design. Defaults to FALSE.
na.rm: logical. Should missing values be removed? Defaults to FALSE.
R: the number of bootstrap replicates. Usually this will be a single positive integer. For importance resampling, some resamples may use one set of weights and others use a different set of weights. In this case R would be a vector of integers where each component gives the number of resamples from each of the rows of weights. See boot.
formula: a formula of the form lhs ~ rhs where lhs is a numeric variable giving the data values and rhs a factor with two levels giving the corresponding groups.
data: an optional matrix or data frame (or similar: see model.frame) containing the variables in the formula formula. By default the variables are taken from environment(formula).
subset: an optional vector specifying a subset of observations to be used.
na.action: a function which indicates what should happen when the data contain NAs. Defaults to getOption("na.action").
...: further argument to be passed to or from methods.

Author

Andri Signorell <andri@signorell.net>

Details

This function collects code from two sources. The classical confidence interval is calculated by means of t.test. The bootstrap intervals are strongly based on the example in boot.

Examples

Run this code

x <- d.pizza$price[d.pizza$driver=="Carter"]
y <- d.pizza$price[d.pizza$driver=="Miller"]

MeanDiffCI(x, y, na.rm=TRUE)
MeanDiffCI(x, y, conf.level=0.99, na.rm=TRUE)

# the different types of bootstrap confints
MeanDiffCI(x, y, method="norm", na.rm=TRUE)
MeanDiffCI(x, y, method="basic", na.rm=TRUE)
# MeanDiffCI(x, y, method="stud", na.rm=TRUE)
MeanDiffCI(x, y, method="perc", na.rm=TRUE)
MeanDiffCI(x, y, method="bca", na.rm=TRUE)

# the formula interface
MeanDiffCI(price ~ driver, data=d.pizza, subset=driver %in% c("Carter","Miller"))

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