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Factors are used for qualitative variables. It's a variable taking a finite number of values like an answer: $Yes$ or $No$. The factor function will create a vector with levels. You will be able to use this later, for instance for grouping those that answered $Yes$ and those who answered $No$.

# format: factor(v, levels = levels)
f <- factor(c("yes", "yes", "no", "idk", "yes","no"), levels = c("yes","no", "idk"))
# [1] yes yes no  idk yes no
# Levels: yes no idk

# implicit levels (unique values), per alphabetical order
f <- factor(c("yes", "yes", "no", "idk", "yes","no"))
# [1] yes yes no  idk yes no
# Levels: idk no yes


You can get the levels with levels (😄).

# [1] "yes" "no"  "idk"

You can get a variable drop a level with droplevels.

droplevels(f, "idk")
# [1] yes  yes  no   <NA> yes  no
# Levels: yes no

You can use this trick to add a level

# set levels properpies for f
# as the previous levels vector concatenated with "???"
levels(f) <- c(levels(f), "idk")
# [1] "yes" "no"  "idk"