calc_ttest: Calculate a t-test and summary statistics

In emilelatour/lamisc: Latour's Little Helpers

Calculate a t-test and summary statistics

Description

Function to return tidy results for a t-test along with the summary statistics if the groups. Fashioned after the t-test output in Stata and the details given there. Also, if requested, checks for equal variance and Cohen's d are returned.

Calculate a t-test (immediate form) and summary statistics. Similar to calc_ttest() except that we specify summary statistics rather than variables and data as arguments.

Function to return tidy results for a t-test along with the summary statistics if the groups. Fashioned after the t-test output in Stata and the details given there. Also, if requested, checks for equal variance and Cohen's d are returned.

Usage

calc_ttest(
  data,
  var,
  by = NULL,
  mu = 0,
  paired = FALSE,
  var_equal = FALSE,
  df_form = "Satterthwaite",
  conf_level = 0.95,
  check_variance = FALSE,
  reverse_groups = FALSE,
  show_cohens_d = FALSE,
  show_alternative = FALSE,
  fmt_res = FALSE,
  accuracy = 0.1,
  ...
)

calc_ttest_i(
  n1,
  m1,
  s1,
  n2 = NULL,
  m2 = NULL,
  s2 = NULL,
  mu = 0,
  var_equal = FALSE,
  df_form = "Satterthwaite",
  conf_level = 0.95,
  check_variance = FALSE,
  reverse_groups = FALSE,
  show_cohens_d = FALSE,
  show_alternative = FALSE,
  fmt_res = FALSE,
  accuracy = 0.1,
  ...
)

Arguments

data	A data frame or tibble.
var	A (non-empty) numeric vector of data values.
by	A factor (or character) with two levels giving the corresponding groups.
mu	A number indicating the true value of the mean (or difference in means if you are performing a two sample test).
paired	A logical indicating whether you want a paired t-test.
var_equal	logical variable indicating whether to treat the two variances as being equal. If TRUE then the pooled variance is used to estimate the variance otherwise if FALSE then unequal variance is assumed and the Welch (or Satterthwaite) approximation to the degrees of freedom is used.
df_form	If unequal variances are assumed, then specify to use "Welch" or "Satterthwaite" (default) approximation to the degrees of freedom. R t.test documentation says it uses "Welch" but actually they use "Satterthwaite". At least, "Welch" in R matches "Satterthwaite" in Stata.
conf_level	Confidence level of the interval. Default level is 0.95.
check_variance	A logical whether to return some tests of homogeneity of variances. Bartlett's, Levene's, Fligner, and check of the ratio of the standard deviations.
reverse_groups	If TRUE, then uses forcats::fct_rev() to reverse the order of the groups being compared. Default is FALSE.
show_cohens_d	Logical whether to return the effect size, Cohen's d. T-test conventional effect sizes, proposed by Cohen, are: 0.2 (small effect), 0.5 (moderate effect) and 0.8 (large effect) (Cohen 1998, Navarro (2015)). This means that if two groups' means don’t differ by 0.2 standard deviations or more, the difference is trivial, even if it is statistically significant.
show_alternative	Logical whether to show alternative hypothesis notation in the test results. Default is FALSE.
fmt_res	Logical whether to format estimates and p-values for the summary_stats, hypothesis_tests, tests_of_homogeneity_of_variance, and variance_check.
accuracy	A number to round to. Use (e.g.) 0.01 to show 2 decimal places of precision. If NULL, the default, uses a heuristic that should ensure breaks have the minimum number of digits needed to show the difference between adjacent values.
...	Additional arguments passed to scales::number()
n1	Sample size of group 1
m1	Mean for group 1
s1	Standard deviation for group 1
n2	Sample size of group 2 (if applicable)
m2	Mean for group 2 (if applicable)
s2	Standard deviation for group 2 (if applicable)

Value

A list

• summary_stats - Summary statistics. Estimates and confidence intervals.

• difference - A statement of the Null hypothesis.

• method - What test is being used. Character string.

• hypothesis_tests - Test statistics, p-values, estimates and confidence intervals for two-sided and one-sided tests.

• tests_of_homogeneity_of_variance - A few difference checks for homogeneity of variances. Levene's test and the F test are fragile w.r.t to normality. Don't rely on these. Levene's test is an example of a test where alpha should be set quite high, say 0.10 or 0.15. This is because Type II error for this test are more severe (claim equal variances when they are not). F-test: Compare the variances of two samples. The data must be normally distributed. Bartlett’s test: Compare the variances of k samples, where k can be more than two samples. The data must be normally distributed. The Levene test is an alternative to the Bartlett test that is less sensitive to departures from normality. Levene’s test: Compare the variances of k samples, where k can be more than two samples. It’s an alternative to the Bartlett’s test that is less sensitive to departures from normality. Fligner-Killeen test: a non-parametric test which is very robust against departures from normality.

• variance_check - A variance check. If the ratio of SDmax and SDmin is less than 2 (SDmax / SDmin < 2) the the pooled estimate of the common variance can be used. (Use var_equal = TRUE). If the ratio of population standard deviations is "close" to 3, then the test can suffer if the sample sizes are dissimilar (even worse if it's the smaller sample size has larger variance). If distributions are slightly skewed, then skewness should be in the same direction and sample sizes should be nearly equal to minimize the impact of this problem. For practical purposes, t-tests give good results when data are approximately symmetric and the ratio of sample standard deviations doesn't exceed 2 (e.g. SDmax / SDmin < 2).

A list

Examples

library(dplyr)
library(tibble)
library(tidyr)

fuel <- tibble::tribble(
  ~mpg, ~treated,
  20L,       0L,
  23L,       0L,
  21L,       0L,
  25L,       0L,
  18L,       0L,
  17L,       0L,
  18L,       0L,
  24L,       0L,
  20L,       0L,
  24L,       0L,
  23L,       0L,
  19L,       0L,
  24L,       1L,
  25L,       1L,
  21L,       1L,
  22L,       1L,
  23L,       1L,
  18L,       1L,
  17L,       1L,
  28L,       1L,
  24L,       1L,
  27L,       1L,
  21L,       1L,
  23L,       1L
)


#### One sample t-test --------------------------------

calc_ttest(data = fuel,
           var = mpg)


calc_ttest(data = fuel,
           var = mpg,
           mu = 20)

calc_ttest(data = fuel,
           var = mpg,
           mu = 20,
           show_cohens_d = TRUE)


calc_ttest(data = fuel,
           var = mpg,
           mu = 20,
           fmt_res = TRUE)


#### Paired t-test --------------------------------

calc_ttest(data = fuel,
           var = mpg,
           by = treated,
           paired = TRUE)


#### 2-sample t-test --------------------------------

calc_ttest(data = fuel,
           var = mpg,
           by = treated,
           paired = FALSE)

calc_ttest(data = fuel,
           var = mpg,
           by = treated,
           paired = FALSE,
           check_variance = TRUE)

calc_ttest(data = fuel,
           var = mpg,
           by = treated,
           paired = FALSE,
           check_variance = TRUE,
           fmt_res = TRUE)


calc_ttest(data = fuel,
           var = mpg,
           by = treated,
           df_form = "Satterthwaite",
           paired = FALSE)

# Compare to Base R
with(fuel, t.test(mpg ~ treated))


calc_ttest(data = fuel,
           var = mpg,
           by = treated,
           df_form = "Welch",
           paired = FALSE)
#### Immediate form --------------------------------

# One sample t-test
calc_ttest_i(n1 = 24, m1 = 21.88, s1 = 3.06)

# Two sample t-test (only available for unpaired)
calc_ttest_i(n1 = 12, m1 = 21.0, s1 = 2.7,
             n2 = 12, m2 = 22.75, s2 = 3.3)

emilelatour/lamisc documentation built on April 9, 2024, 10:33 a.m.