Descriptive statistics by group in R

Problem

We'd like to report descriptive statistics in R by a grouping variable and subsetting the output statistics.

Solution

We will use the data frame iris, columns Sepal.Length and Sepal.Width and grouping by Species. In our example, we want to return the mean, the standard deviation, the skewness and kurtosis.

Subset of descriptive statistics by group

library(psych)
# Variables by index
d <- describeBy(iris[1:2], group = iris$Species)
# Two options to subset the statistics:
lapply(d, "[", , c(3, 4, 11, 12))
lapply(d, subset, , c(3, 4, 11, 12)) 

# Variables by name
i <- match(c("Sepal.Length", "Petal.Length"), names(iris))
d <- describeBy(iris[i], group = iris$Species)
lapply(d, subset, , c("mean", "sd", "skew", "kurtosis")) 

$setosa
             mean   sd skew kurtosis
Sepal.Length 5.01 0.35 0.11    -0.45
Sepal.Width  3.43 0.38 0.04     0.60

$versicolor
             mean   sd  skew kurtosis
Sepal.Length 5.94 0.52  0.10    -0.69
Sepal.Width  2.77 0.31 -0.34    -0.55

$virginica
             mean   sd skew kurtosis
Sepal.Length 6.59 0.64 0.11    -0.20
Sepal.Width  2.97 0.32 0.34     0.38

Subset of descriptive statistics without grouping

# Seleccionamos las columnas deseadas de la tabla
d <- describe(iris[1:2])
# Subsetting output statistics
d[, c(3, 4, 11, 12)]

             mean   sd skew kurtosis
Sepal.Length 5.84 0.83 0.31    -0.61
Sepal.Width  3.06 0.44 0.31     0.14

References

• Descriptive statistics in R