A data frame where each row corresponds to a variable and each column represents a summary statistic:
number of non-missing observations (n), arithmetic mean (Mean),
standard deviation (Std.Dev), median (Median), minimum (Min),
maximum (Max), first quartile (25th), third quartile (75th),
sample skewness (Skew, from moments::skewness), and sample kurtosis
(Kurtosis, from moments::kurtosis).