What is the probability that in a household of five, two people will have the same birthday?
Fellow English majors, stop rolling your eyes and read on.
The Birthday Problem is a well-known probability exercise that’s been around since who-knows-when. I’m sure someone in the math department could calculate the probability of me guessing the answer, but as the T-shirt says, “English Major. You do the math!”
What the Birthday Problem establishes is that the odds are in favor of two people having the same birthday in a random group of just 23 people. Math teachers love to dazzle students by wagering that two people in the room have the same birthday. As long as the class has at least 22 students (the teacher makes 23) the odds are in the teacher’s favor. Just barely, with a probability of 50.7%, but in his favor nonetheless. With 40 people in that lecture hall, the probability goes up to 89.1% and he becomes Professor Marvel.
There is a formula that explains that, but it uses lines and symbols and squiggly things that look to me more like Franco-Cantabric cave paintings than a math problem. If you care about the formula, you already know about the Birthday Problem, so let’s just move on. Someday you can explain it to me and I will explain gerundives to you. All I care about is that it’s an easy win at the tavern because all the other English majors think the odds are in their favor if there are fewer than 367 people present.
But even I know that with five people in the house there isn’t much chance that two of us have the same birthday. As the cave paintings prove, the probability is a mere 2.7%.
And yet, here we are. It’s our birthday.
My birthday is especially easy to remember: 6-1-61. (Don’t bother. It’s not my PIN). Vito, the Russian hockey player who lives with the four Streulis, was born on 6-1-98. (Not his PIN either). What are the odds?
Last year we spent our birthday at Remington Park because Vito had never seen a horse race and wanted to check it out. He brought friends, I brought friends. Then we tried to explain to three Russian hockey players how parimutuel wagering works. That means telling them that all the bettors are wagering against each other, not against the racetrack. And that the odds on any particular horse to win are established by how much of the wagered money has been placed on that particular horse, so it’s a reflection of how confident people are in that horse not how fast the horse is. And yes, it could be just one really big bet that made that horse the favorite, perhaps made by someone with insider knowledge you’d want to follow. Then again, it might have been placed by the drunk guy in the fedora wearing the bird of paradise aloha shirt and dollar-store flip-flops.
“Which horse do I bet on?”
“The one you think is going to win.”
“Which one is that?”
“I don’t know.”
“What is this exacta?”
“You have to pick the horse that comes in first and the horse that comes in second in the right order.”
“That is hard?”
“Yes, that is hard.”
“What is this trifecta?”
“You have to pick first, second and third. In the right order.”
“Very hard. It’s hard enough to pick which one is going to come in first.”
“Okay. What does it mean to box the superfecta? How much can you win?”
“I was an English major. You do the math.”