Data frame with the EQ-5D, grouping, id and follow-up columns
name_id
Character string for the patient id column
name_groupvar
Character string for the grouping column
names_eq5d
Character vector of column names for the EQ-5D dimensions
name_fu
Character string for the follow-up column
levels_fu
Character vector containing the order of the values in the follow-up column.
If NULL (default value), the levels will be ordered in the order of appearance in df.