A character vector with length equal to the rows of df, including
one letter or underscore for each column of df. For instance, if df
has one column for each of the eight options of the Nacho Craving Index
example instrument (https://libguides.du.edu/c.php?g=948419&p=6839916),
with a row containing the values "Chips" (checked), "Yellow cheese"
(unchecked), "Orange cheese" (checked), "White cheese" (checked), "Meat"
(checked), "Beans" (unchecked), "Tomatoes" (unchecked) and "Peppers"
(checked), then the character string corresponding to that row will be
"a_cde__h". The underscores represent that the options for "Yellow
cheese", "Beans", and "Tomatoes" were left unchecked.