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Last week I posed a question: what five cities are home to the greatest number of my Harvard classmates?
Without a doubt, this was the hardest quiz ever on this blog. Over 1,000 guesses were made; the first 851 of these guesses were wrong. (Actually, blog reader Len sort of had the right answer earlier, but it is totally obvious if you’ve ever seen the red book that is my data source, that he cheated.)
Not until John F. came along with comment number 821 did someone end the misery by getting all five cities correct (with number of my classmates in parentheses):

• New York (approx. 250)
• San Francisco (55)
• Los Angeles (38)
• Cambridge (35)
• Washington, DC (31)

Other cities that were close to the top five were Seattle with 22 and London with 21.
What made this question so hard? My crack researchers Jordan Ou and Laura Rivera analyzed the guesses made by the first 250 guessers. Here is how often various cities were named

• New York (233)
• Boston (178)
• Washington, DC (153)
• Chicago (126)
• Los Angeles (104)
• San Francisco (93)
• London (87)
• Houston (24)
• Seattle (20)
• Cambridge (17)
• Shanghai (15)
• Hong Kong (13)

So what really hurt people was including Boston and Chicago which, surprisingly, are home to only 20 and 18 of my classmates respectively. Hardly anyone guessed that so many of my classmates would stay right in Cambridge.
I was surprised how few Harvard alums live in Chicago. Amazingly, there are almost as many of my classmates in Oakland (14) as there are in Chicago, and yet not a single person in the first 250 guessed Oakland.
Another thing that is surprising, given the answers people gave, is how long it took to get a correct answer. Imagine you had an urn filled with balls that each had one city’s name on it, with the balls in proportion to the frequency with which blog readers named each city (e.g. 233 New York balls,
178 Boston balls, etc.). Now randomly pull balls out of that urn until you get five unique cities. The probability that the five balls that you end up with will match the correct five cities, by my calculations, is about 1 in 166. So the fact that it took over 800 guesses to get the right answer suggests that there was a negative correlation between some of the winning cities. So, for instance, guessers who thought people didn’t move far tended to list both Cambridge and Boston, but rarely mentioned Los Angeles or San Francisco.
And to all of you who guessed foreign cities, the simple fact is that hardly any of my classmates live abroad, except for those who were foreigners to begin with (most of the Canadians went back home) or moved to London. Eight of my classmates are in France and Hong Kong, but just three live in mainland China. The most unlikely place I could find one of my classmates living: Albania.