Marge:
"suppose the wisest thing is to regard the printed broadsides as a snapshot of a song, showing how it was sung at a particular time,"
Ballad scholar David Buchan went as far as to suggest that, within the ballad singing tradition there were no set texts but just plots and a number of commonplaces and conventions.
He proposed that a ballad singer took these and re-composed the song each time he or she sang it.
I don't believe he made his case completely, but it's an inriguing thought.
Jim Carroll