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Correlation Causation

M. C. DeMarco
United States
Boston
Massachusetts
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Long ago, BGG had a user-to-user correlation tool. So long ago, in fact, that user
Mikko Saari
Finland
Tampere
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http://www.lautapeliopas.fi/ - the best Finnish board game resource!
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wrote a replacement for it back in 2006. A few bugs were knocked out of it early on and features were added over the years, but in my recent experience it's still a bit flaky: it doesn't always respond, and the error message you get does not always reflect the actual problem. More problematically, it is very demanding in terms of numbers of games you've rated, and it doesn't use the most appropriate correlation coefficient.

So, when I compared myself to all BGG users, even at the minimum threshold of 50 games, I only got the BGG average rating compared to my 112 games rated at the time. Those results looked something like this:

User: BoardGameGeek average rating
Corr: 0.52
#: 112

Despite the lack of compatible users, I managed to compare my ratings to those of the incomparable
Russ Williams
Poland
Wrocław
Dolny Śląsk
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I have preserved the results here for others like me who don't rate much and may have trouble finding anyone on BGG who has rated the same 50 games as themselves.

First, the summary:

Correlation: 0.27

Average for fiddly_bits: 7.15
Average for russ: 6.63

Next, the pretty picture, captioned "The graph shows your ratings along the bottom and the other person's ratings along the side. The darker the dot, the more games fall in that spot."



Finally, the source data: games both I and Russ rated, along with the (rounded, usually up) difference in our ratings. You can sort by difference (in theory, by clicking on the word Diff, but in practice I only got it working by adding &sort=diff to the url).



(I saved this as an image because the margin was too small to reformat the data.)

I rated a bunch more games (I didn't start out with 112) hoping to get better data for this post, but Russ is still my only known game soulmate. If I were feeling ambitious, I would search for more soulmates by writing a tool that used a better correlation statistic.
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