aggregate               Aggregate data based on given grouping.
correlation_threshold   Remove redundant variables.
count_na_rows           Count the number of 'NA's per variable.
covariance              Compute covariance matrix and vectorize.
drop_na_columns         Remove variables with NA values.
drop_na_rows            Drop rows that are 'NA' in all specified
                        variables.
extract_subpopulations
                        Extract subpopulations.
generalized_log         Generalized log transform data.
generate_component_matrix
                        A sparse matrix for sparse random projection.
normalize               Normalize observation variables.
replicate_correlation   Measure replicate correlation of variables.
sparse_random_projection
                        Reduce the dimensionality of a population using
                        sparse random projection.
svd_entropy             Feature importance based on data entropy.
transform               Transform observation variables.
variable_importance     Measure variable importance.
variable_select         Select observation variables.
variance_threshold      Remove variables with near-zero variance.
whiten                  Whiten data.
