Removes "Do not know" and other similar words from factor levels in data frame
remove_all_dont_know(x, dk = NULL, message = TRUE)List or data frame
Character vector, containing search terms, e.g. c("Do not know", "DK").  These terms will be replaced by NA. If NULL, defaults to c("I don't know", "Don't Know", "Don't know", "Dont know" , "DK")
If TRUE, displays message with the number of instances that were removed.
A data frame
hasDK() and removeDK()
Other Functions to clean data: encToInt,
  fix_common_encoding_problems,
  fix_levels_01_spss,
  has_dont_know, intToEnc,
  leveltest, remove_dont_know