Birthday

Thanks to all those who wished me a happy birthday—and to Ed Brayton, happy birthday in return!

Incidentally, here’s Richard Dawkins on the birthday coincidence:

If you have a roomful of only 23 people, mathematicians can prove that the odds are just greater than 50 percent that two of them will share the same birthday…. It’s easier to calculate the odds that there is not a pair of shared birthdays in the room. Let’s pretend that leap years don’t exist, and suppose you and I are among the 23 people in a room. My birthday is 26 March. I don’t know when yours is but since there are 364 days that are not 26 March, the odds are 364/365 (0.997) that your birthday is not mine. But the pairing of you with me is only one of many pairings that we could imagine in our roomful of 23 people. We have to multiply 364/365 by itself for each pairing. How many pairings? A first guess is 23 x 23 (=529) but this is clearly too many. It allows each person to be paired with himself…. So we must at least subtract 23 from our preliminary list of possible pairings, which gives us (23 x 23) – 23 = 506. And obviously if I share my birthday with you, you must share your birthday with me. In other words it counts each pairing twice. So we must halve our 506, giving 253 as the number of pairings that we must consider…. 364/365 multiplied by itself 253 times yields a number very close to 0.5. This is the chance that there will not be any shared birthdays in the room. So there’s an approximately even chance that at least one pair of individuals in a committee of 23 will share a birthday…. If you do the equivalent working for 30 people…the chance of a couple of shared birthdays here is about 70 percent.

Richard Dawkins, Unweaving The Rainbow 153-54 (1998).

There are 79 links on my blogroll which have also linked to me, and thus it’s reasonable to suppose some 79 bloggers who occasionally read Freespace. So the chances that one of them wouldn’t share my birthday is extremely small—practically none. I don’t want to do the math (0.997 times itself 3,081 times), but let’s assume that the answer is that two bloggers on the blogroll are going to share my birthday. Brayton’s chance of being one of them is, of course, one out of about 40.

My folks came up and we went up to Tahoe, which was not nearly as pretty as it usually is, due to the smoke from the forest fires. But at least it wasn’t crowded at all. (Today it’s rained all day, which I’m sure is helping the firemen.) We just lazed around, not doing much. They gave me a cool machine to identify various bird calls, and a book of old photos from my home town. But, although I gave them ample opportunity, they did not get me an electric guitar. I was patient, I gave them all the time they needed….anyway, so I got it for myself on the way home from lunch, and I’ve started trying to learn “Hell Hound On My Trail,” as performed by Eric Clapton.

Update: Oops. Froggie points out that I should not have halved the 79. My mistake. And of course everyone was born in October; it’s nine months after Valentine’s Day.

Update 2: Inclination to Criticize has an even more thorough explantion of my mathematical error. I was looking for people who shared any birthday, when I ought to have been looking for people who shared one particular birthday, the odds of which are quite a bit smaller than the odds of sharing any day in common.