Has there been any kind of analysis about potential biases? From the top of my head I can think of the fact that Spotify was not available in every country from the start. Also, not every person on the world streams their music. Does this data include record sales for examlpe? Does it normalise the data somehow taking into account the percentage of the population that streams music versus other means of consuming music?

Or is it just plain and simple data gathered from Spotify?

I'm not saying that there is anything wrong/bad about the results. But without knowing the details on how the data is collected, it's hard to read anything from the results.