They studied 50 years of UK top 40 charts, totting all the top five and bottom 10 hits. This allows the algorithm to rate a new, uncharted tune, by comparing its weights to the top and bottom echelons of the chart.
Depressing, yes. Surprising, not really. I got empirical with head researcher and senior A.I. lecturer at Bristol Universtiy, Tijl De Bie, to find out more.