Photo: surlygirl

Situations arise from time to time where you are in a room with a whole bunch of strangers. They key to making friends and the opposite sex swoon? Probability.

What are the odds that two people in the room have the same birthday? Memorize some of these numbers so that you can spout them off, I guarantee you will be the coolest guy in the room – 9 people = 10%, 13 = 20%, 15 = 25%, 18 = 35%, 23 = 51%, 57 = 99%, 366 = 100%.

It’s important that you mention the assumptions before your new worshipers start poking holes in you numbers – random distribution of birthdays, no leap years, no twins, etc. Then you can go into what is interesting – with only 23 people in the room, odds are two people share a birthday. It only takes 57 people to jump to 99% probability, but to get to 100% you need to have 366 people.

Now to really make their mouths drop. How many people would it take for the odds to be in your favor for someone having the same birthday as you? 253. They may question you because this is much higher than 365/2, but stick to your guns and point out that two of the other people could have the same birthday. How many people would it take for two to likely have a birthday within a week of each other? Only 7. Ya, but a random distribution is a hell of an assumption. Oh really? Actually the odds are slightly more in your favor because birthdays tend to clump – summer babies, C-sections aren’t on weekends, and so on.

Here’s one to set up on a tee for them: what are the odds that two people have the same half birthday? If they can figure this out, you may just have found your soulmate.

If you have the same half bday, doesn’t that just mean you have the same bday too? So that’s just the same as everything you were just saying. Am I missing something?

No, you are not missing anything – I set it up on the tee for you and you knocked it out of the park! In this part of the hypothetical conversation I setup, the goal wasn’t to stump you or make you do mental math – I want you to feel smart or at least involved in the conversation.

Lindberg is your soulmate!

Oh I see what you did there. Nice. I’m used to doing this every week: http://richardwiseman.wordpress.com/2010/09/03/its-the-friday-puzzle-75/

You gotta think outside the box. find the trick. do all the mental calisthenics. it can get frustrating…

What are the odds that two people from different states, would end up working for the same company, end up in the same plant, share the same birthday, and share the same first name. On top of all this, before we knew anything else, other than first name…Had automatic chemistry? Is this a soulmate…Stumped and stunned?

It would take a PhD is Statistics to figure that out! But maybe I can help a little?

The website howmanyofme.com tells me that there are 230,957 Erins in the US. That is 1 in 1330 people. The odds that a given Erin has your birthday is roughly 1 in 365. I don’t know what state you are from, but if we assume all states have the same number of people, the odds this other Erin is from another state is 49 out of 50. So one in a half million people in the US will be named Erin, have your birthday, and be from out of state!

What are the odds that one of these people would work at the same plant and have automatic chemistry? I am stunned!

That is so cool… Thanks… I thought it would be a pretty rare situation…. I never believed in signs…but starting to rethink that…lol…