Monday, November 6, 2017

Metal and sunshine

One of the most celebrated examples in the Visual Trumpery lecture is this map of concentration of Heavy Metal bands per million people in Europe (source):

I use this map to discuss how to verify a source (spoiler: I love the map, and the data on it looks OK to me.) During the talk I often joke that this confirms something that, as a hard rock fan, I already guessed: overall, the less sunlight a country gets, the more metal bands it tends to have.

The audience loves this joke, but is it true? It turns out that, at least at a first glance, it may not be or, at least, it needs some caveats. Here's a quick ggplot2 of the relationship between the average yearly sunshine duration in the capitals of most of these countries and the concentration of metal bands (Note: I used the capital as a rough proxy, being very aware that many of these countries are big, so that the amount of sunlight within them varies a lot. In Spain, for instance, the South is very sunny, but Northern regions such as Galicia, where I was born, are cloudy, rainy, dark —and maybe very metal):

So my hunch is tentatively dubious, at least until we get more detailed data and think carefully about it: the relationship is clearly not linear and, even if that were the case, correlation isn't that strong. Besides, the graph reveals some intriguing features, such as at least three clusters of countries. The group on the left is largely made of Western, non-Mediterranean countries; the one in the middle is mostly Eastern and South-Eastern countries; and the last group, on the right, is Southern Mediterranean countries (Spain, Portugal, Greece, Turkey, and Italy.)

If we isolate the first group, with the exception of an outlier (Iceland) the relationship is positive: the more sunlight, the more metal bands. And if we ignore the four outliers on the Y-axis from the overall mix (Finland, Norway, Sweden, and Iceland,) there's barely any relationship. See graphs below.

What can we learn from all this? Not much, I guess, other than that even silly jokes ought to be verified and presented with plenty of caveats.