By far, the largest category in the UGA music professor's study is Normal. Most people can differentiate between songs but not whether notes are sharp or flat. This partially explains the popularity of singers such as Bob Dylan, Neil Young and Tom Petty. Most people can't tell that their singing sucks.
Nobody ever accused Bob Dylan of being a singer! His fans are more about what he sings than his singing - now a guy like Frank Sinatra could sing the phone book and sound great.