ttest: T-test with Improved Layout

Description

Performs a one- or two-sample t-test using data. In the two-sample case, the user can specify whether or not observations are matched, and whether or not equal variances should be presumed.

Usage

ttest(
  var1,
  var2 = NA,
  by = NA,
  geom = FALSE,
  null.hypoth = 0,
  alternative = "two.sided",
  var.eq = FALSE,
  conf.level = 0.95,
  matched = FALSE,
  more.digits = 0
)

Value

a list of class ttest. The print method lays out the information in an easy-to-read format.

tab: A formatted table of descriptive and inferential statistics (total number of observations, number of missing observations, mean, standard error of the mean estimate, standard deviation), along with a confidence interval for the mean.
df: Degrees of freedom for the t-test.
p: P-value for the t-test.
tstat: Test statistic for the t-test.
var1: The user-supplied first data vector.
var2: The user-supplied second data vector.
by: The user-supplied stratification variable.
par: A vector of information about the type of test (null hypothesis, alternative hypothesis, etc.)
geo: A formatted table of descriptive and inferential statistics for the geometric mean.
call: The call made to the ttest function.

Arguments

var1: a (non-empty) numeric vector of data values.
var2: an optional (non-empty) numeric vector of data.
by: a variable of equal length to that of var1 with two outcomes. This will be used to define strata for a t-test on var1.
geom: a logical indicating whether the geometric mean should be calculated and displayed.
null.hypoth: a number specifying the null hypothesis for the mean (or difference in means if performing a two-sample test). Defaults to zero.
alternative: a string: one of "less", "two.sided", or "greater" specifying the form of the test. Defaults to a two-sided test.
var.eq: a logical value, either TRUE or FALSE (default), specifying whether or not equal variances should be presumed in a two-sample t-test. Also controls robust standard errors.
conf.level: confidence level of the test. Defaults to 0.95.
matched: a logical value, either TRUE or FALSE, indicating whether or not the variables of a two-sample t-test are matched. Variables must be of equal length.
more.digits: a numeric value specifying whether or not to display more or fewer digits in the output. Non-integers are automatically rounded down.

Details

Missing values must be given by NA to be recognized as missing values.

Examples

Run this code


# Read in data set
data(psa)
attach(psa)

# Perform t-test
ttest(pretxpsa, null.hypoth = 100, alternative = "greater", more.digits = 1)

# Define new binary variable as indicator
# of whether or not bss was worst possible
bssworst <- bss
bssworst[bss == 1] <- 0
bssworst[bss == 2] <- 0
bssworst[bss == 3] <- 1

# Perform t-test allowing for unequal
# variances between strata -#
ttest(pretxpsa, by = bssworst)

# Perform matched t-test
ttest(pretxpsa, nadirpsa, matched = TRUE, conf.level = 99/100, more.digits = 1)