The Birthday Problem: Or, We’re Terrible at Estimating Probability

While I’m away from the computer, here’s a guest post from one of my favorite people, Caren Gussoff ( writing about her new book. Caren was in my Clarion West class and is a terrific writer. I haven’t got my hands on the book yet but I do know: BOB is a character in the book.

Humans are naturally terrible at estimating the probability of something happening. There are many theories as to why: it’s a side effect of subjective consciousness, that we seek a “confirmation bias” to inject meaning into the random data that we move through on a daily basis; a trait somehow favored in our evolution that’s kept us safe and viable (fearful or emotional events leave a bigger imprint on us); or simply that few folks receive an adequate education in maths when we are young enough for it to carve into our thinking. It fascinates me that our intuition about how frequently (or infrequently) events can and do happen — for instance, thinking of a song right before hearing it on the radio — is wrong (we think it’s an “eerie coincidence,” while, in fact, the actual probability of such a thing happening is higher than we think). In fact, this skew is the theme of my latest novel, The Birthday Problem (Pink Narcissus Press, 2014).

We personalize everything … which makes sense, because, given our consciousness, everything happens to us (the individual) or in relationship to us (someone we know, don’t know, and not-me). I wanted to write a book that relied on coincidence; in The Birthday Problem, every character has a relationship to every other character in some way or another. They are a web of people all affected by the same tragedy — malfunctioning nanobots that cause symptoms of mental illness. But their interconnectivity was how I played with the phenomena of poor probability judgment and whether it felt possible that all of these very different characters could be so closely related.

The title refers to a classical paradox that, IMHO, nicely sums up our failings in understanding probability. The birthday problem asks how many people need to be in a room together for there to be a 50% (or greater) probability that two of them share the same birthday (month and date).

Unless you already know the answer, whatever you are thinking right now is wrong. Now, it feels like there need to be a LOT of people in a room for there to be a 50/50 probability two of them share the same birthday. You may even think, quite logically, that it would require at least 365 or more. But really, it just takes 23 people in one room to get a 50% probability that two of them will share the same birthday.

There’s great explanation of the math here: Understanding the Birthday Problem.

For less number-oriented thinkers, this explanation is quite tidy: The Birthday Paradox.

Interesting, right? Even when you see the math, it still feels wrong. And that unsettling feeling is what I tried to recreate in The Birthday Problem.

The Birthday Problem is available at Pink Narcissus Press, Amazon, B&N, and in stores everywhere.

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