First we’ll look at the relative popularity of different chords based on the frequency that they appear in the chord progressions of popular music. Then we’ll begin to look at the relationship that different chords have with one another. For example, if a chord is found in a song, what can we say about the probability for what the next chord will be that comes after it?

This is a three part series and is very interesting. I can’t imagine what must have gone into analyzing so many songs.