For example, in 1961, Bobby Lewis’s Tossin’ and Turnin’ spent 7 weeks at #1. For all intents and purposes, Bobby Lewis was the Beyonce of 1961. Yet, have you heard of it? Do you know who Bobby Lewis is?

Meanwhile, Etta James’ debut album dropped the same year, with At Last peaking on Billboard at #68.

Music historians will regard Bobby Lewis as a pioneer in rock and roll and R&B, yet whatever led to Tossin’ and Turnin’s popularity in 1961 has faded over time. His music, for countless reasons, didn’t persevere in the same way as Etta James’.

One hypothesis: Tossin’ and Turnin’s success had more to do than just the song...perhaps Bobby Lewis was a huge personality. Great looks. Amazing dancer. When we examine pop hits, popularity is so much more than song quality.

But future generations don’t remember Bobby Lewis’s dancing and good looks. Spotify only catalogues his music. And unfortunately, that quality didn’t endure in the same way as At Last. (And of course, we have not even considered the role of covers, samples, and movie soundtracks, etc. – a future project to undertake).

And for this reason, it will be weird to hear future generations reverently listen to groups such as Nickelback – the kids only know their music, not what they culturally stood for in 2015.

Thus far we’ve only examined two sets of data points: popularity at release (Billboard) and popularity today (Spotify).

Yet, we don’t know what happened in-between: a song’s trajectory from peak popularity until today. If we had such data points, we’d be able to understand how major cultural events or changes in taste influenced the trajectory of a song.

With Spotify, we have so much more detail: daily playcounts. We can understand a track's decay rate from its peak popularity: whether it’s quickly fading from culture or holding steady to be played by future generations.

Let’s look at the trajectory of the most popular tracks from 2013 and how they're faring today, in 2015.