iTunes allows users to rate songs from 1 to 5 stars. For more precise ratings, enable the half-star rating option in iTunes. Very useful when a song is not quite worth 4 stars but not 5 stars either. Close iTunes and use the following defaults write command:
defaults write com.apple.iTunes allow-half-stars -bool true
Relaunch iTunes for changes to take effect. To give a half star rating, click and slide the star rating scale until the 1/2 value shows.
Removing the half star rating option moves the song rating to the star below it. To remove the half-star rating option:
defaults delete com.apple.iTunes allow-half-stars
Screencast: