# WelchDegreeOfFreedom: Welch's rectified degree of freedom In superb: Summary Plots with Adjusted Error Bars

 WelchDegreeOfFreedom R Documentation

## Welch's rectified degree of freedom

### Description

When variance across groups are heterogeneous, the Student `t` distribution with `n - 1` df is not the exact distribution. However, \insertCitew47superb, using methods of moments, was able to find the best-fitting `t` distribution. This distribution has degree of freedom reduced based on the sample sizes and the variances of the group tests. The present function returns the rectified degree of freedom

### Usage

``````WelchDegreeOfFreedom(dta, cols, groupingcols)
``````

### Arguments

 `dta` A data frame containing within-subject measures, one participant per line; `cols` A vector indicating the columns containing the measures. `groupingcols` A vector indicating the columns containing the groups.

### Value

df the degrees of freedom rectified according to Welch (1947).

\insertAllCited

### Examples

``````# creates a small data frames with 4 subject's scores for 5 measures:
dta <- data.frame(cbind(
DV.1 = c(3., 6., 2., 2., 5.),
DV.2 = c(4., 5., 4., 4., 3.),
DV.3 = c(2., 7., 7., 8., 6.),
DV.4 = c(6., 8., 4., 6., 5.),
grp  = c(1., 1., 2., 2., 2.)
))
# performs the test (here rectified df = 1.898876)
WelchDegreeOfFreedom(dta, "DV.1","grp")

``````

superb documentation built on May 29, 2024, 8:51 a.m.