This function takes a data frame, identifies columns that are not specified in the exclusion list, and combines them into a new column called 'group'. The original columns used for the combination are then removed. Finally, it returns a data frame with only the 'group', 'variables', 'std_diff', and 'balance' columns.
.combineColumns(df)
A data frame with the combined 'group' column and specified columns.
A data frame containing the columns to be combined.