df: Data frame with the EQ-5D, follow-up and grouping columns
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.
name_groupvar: Character string for the grouping column
eq5d_version: Version of the EQ-5D instrument
country: A character string representing the name of the country. This could be in a 2-letter format, full name or short name, as specified in the country_codes datasets.