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Hank helps us understand the difference between the colloquial meaning of randomness, and the scientific meaning, which is also known as stochasticity. We will learn how, in fact, randomness is surprisingly predictable.

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[intro music] Hank Green: Welcome to another edition of I Don't Think That Means What You Think It Means. I'm almost positive that stochasticity doesn't mean what you think it means, probably because you've never heard of the word. It might not be one of those quotidian sciencey terms that you hear all the time like "quantum" or "Bunsen burner", but it's used a lot in science, and it's used to mean randomness, but not like "OMG that's so random", which in common vernacular means "unpredictable" or "weird". Actually, randomness is surprisingly predictable. A better way to think about randomness is unpredictability happening within a set of predictable rules. One of the best-known examples of our misconceptions about randomness is an experiment from the University of California at Berkeley, where one group of students is asked to flip a coin a hundred times and write down the results -- a hundred heads or tails -- on a chalkboard. Meanwhile, another group of students is asked to just write down what they think the results of a hundred coin tosses might look like, without doing the actual flips; they just write down heads and tails in a way that they think looks random. Then a statistician walks into the room and looks at both chalkboards, and tells the class which team actually flipped the coin. She can tell because the real results display patterns that don't show up in the fake results. In one stretch of coin tosses, for instance, there will be, say, like seven tails in a row. The odds of that happening in 100 flips is actually about 1 in 3 -- it's pretty good -- but the students who fake the results never write down anything like that because it doesn't look random enough. So, in a stochastic system, you can never predict what the exact result will be, but you can forecast the probability that certain sequences will show up, not just with experimentation but with math. Scientists incorporate stochasticity into the study of all kinds of things, from how a gas molecule moves across a flask to how changes in an ecosystem will affect the organisms in it. These kind of events are the result of a whole bunch of different inputs, some of which lead to predictable results, others are totally arbitrary, but when scientists look at that flask or ecosystem, they can understand randomness. So, while that tween at the mall might think that her friend's choice of earrings is "so random", that's actually only true if the choice was due to a set of probabilities that, if known, would allow scientists to accurately chart the probability of the use of those earrings over time, which, you know, they probably could now that I think of it, if they didn't have better things to do, which thankfully they do. So, randomness, or stochasticity in fact, has nothing to do with whether an outcome is expected or improbable. Randomness is in fact surprisingly predictable. And so now you know! Feel free to use this knowledge to alienate all your friends. [tossing coin] Thank you for watching this SciShow Dose. If you wanna keep getting smarter with us here at SciShow, you can go to and subscribe. If you have any questions or suggestions or ideas for us, you can leave them on Facebook or Twitter, or of course in the comments below. I dropped it. The last one. I was gonna tell you what I got. I dropped it again! It's heads. [outro music]