In Celebration of the First Of Three Friday The Thirteenths This Year…

About Kylie Sturgess

Kylie Sturgess is a Philosophy teacher, media and psychology student, blogger at Patheos and podcaster at Token Skeptic. She has conducted over a hundred interviews including artists, scientists, politicians and activists, worldwide.
She’s the author of the ‘Curiouser and Curiouser‘ column at the Committee for Skeptical Inquiry website and travels internationally lecturing on feminism, skepticism, and science.

  • Andrew G.

    Trivia fact: the 13th of the month falls more often on Friday than any other day of the week.

  • Katkinkate

    How can that be true?

  • Andrew G.

    The proof is left as an exercise for the reader.

    (Which is literally where I first encountered it; as a reader exercise in a programming book, “Write a program to prove that the 13th day of the month falls most often on Friday”. I believe the book was Richards and Whitby-Strevens book on BCPL, but I no longer have it handy to check.)

    (And yes, it really is true – it is trivial to prove that the weekday cannot be equally distributed, and the fact that Friday happens to be the most common day is just a matter of counting.)

  • Katkinkate

    I call shenanigans. Since Mr G isn’t prepared to substantiate his claim (and I had nothing else to do), I did a quick check over a 20 year span (1995 to 2014) for the incidences of Friday falling on the 13th of the month. The expected average is 1.7 (12 months divided by 7 days) and the resultant average over the 20 year span was …. 1.7 (34 incidences over 20 years). I’m not surprised, I could see no way there could be any bias as the days are laid out in order every week, month and year. There is no mechanism to give bias to any one day. So your ‘trivia fact’ is in fact a myth.

  • http://freethoughtblogs.com/tokenskeptic Kylie Sturgess

    Hi – I used this site: http://www.crescentmooncycles.com/html/friday_the_13th_dates.html

    Friday, February 13th 2009
    Friday, March 13th 2009
    Friday, November 13th 2009
    Friday, August 13th, 2010
    Friday, May 13th, 2011
    Friday, January 13th, 2012
    Friday, April 13th, 2012
    Friday, July 13th, 2012
    Friday, September 13th, 2013
    Friday December 13th, 2013
    Friday, June 13th, 2014
    Friday, February 13th, 2015
    Friday, March 13th, 2015
    Friday November 13th, 2015…

    There’s more there, if people want to check?

  • Andrew G.

    Heh. I always find that this one is useful for drawing out sloppy thinking and knee-jerk responses.

    First, the proof that the days cannot be evenly distributed: the Gregorian calendar repeats exactly after 400 years (400 years is 400*365 + 97 days, which is divisible by 7 so every 400-year period starts on the same day of the week). 400 years is 4800 months which is not divisible by 7. Therefore an even distribution of days for a regular monthly event is not possible.

    All that remains is to count which days the 13th falls on over that 400-year period, which is trivial in any useful programming language. The result is: Sunday 687, Monday 685, Tuesday 685, Wednesday 687, Thursday 684, Friday 688, Saturday 684. Friday wins.

    If you pick a period shorter than 400 years and count that, you’ll get a different result (e.g. the 30 years from 1995-2014 has more Thursday 13ths than any other day). But that’s entirely dependent on what period you choose to sample; only by counting the whole 400-year cycle can you get the true long-term figure.

  • Kylie Sturgess

    The bigger question – did anyone try folding a cat?

  • Andrew G.

    Typo; 20 years, not 30.

  • Katkinkate

    Sorry Kylie.

  • http://www.freethoughtblogs.com/tokenskeptic Kylie Sturgess

    :) Just wondered if anyone liked the cat!

  • http://itsmyworldcanthasnotyours.blogspot.com WMDKitty

    Sunday 687, Monday 685, Tuesday 685, Wednesday 687, Thursday 684, Friday 688, Saturday 684. Friday wins.

    But the distribution is still roughly equal, so Friday doesn’t really stand out as much as you’re trying to make it.

    Now, if the other numbers were, like, in the 400-500 range, and Friday still had 688, you’d have a point.

    • Kylie Sturgess

      I just want more Fridays, 13th or no – is that bad? :(

    • tm

      so Friday doesn’t really stand out as much as you’re trying to make it

      There’s an example of that sloppy thinking Andrew referred to. His statement was simply

      the 13th of the month falls more often on Friday than any other day of the week

      It’s a simple factual claim, and it’s true. “really stand out as much as you’re trying to make it” is pure projection, reading something into it that isn’t there.

  • http://itsmyworldcanthasnotyours.blogspot.com WMDKitty

    Kylie, I’m usually too busy unfolding my cat from various crazy positions that take up the whole bed. But now that you’ve mentioned it…

    *runs off to find some origami paper*

    Damn. No paper.

    • Kylie Sturgess

      Best origami I’ve seen done was using the butcher’s paper that covered a table in a restaurant. Huge folds, big bird!


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