Knowing What You Don’t Know

… as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don’t know we don’t know.

—U.S. Secretary of Defense Donald Rumsfeld

This is clearly tortured language, and the meaning of the sentence is obscured by that. There’s no reason for the repetitive use of the same word, and the secretary might have been clearer if he had said instead, “There are things we know, things we are aware that we do not know, and some things we aren’t even aware that we don’t know.” There’s a fourth possibility, of course—things we know that we aren’t aware we know. You’ve probably experienced this—someone asks you a question and you answer it, and then say to yourself, “I’m not even sure how I knew that.”

Either way, the fundamental point is sound, you know? What will really hurt you, and cause untold amounts of damage and inconvenience, are the things you think you know but don’t (per Mark Twain’s/Josh Billings’s epigraph at the beginning of this book), and the things that you weren’t even aware of that are supremely relevant to the decision you have ahead (the unknown unknowns). Formulating a proper scientific question requires taking an account of what we know and what we don’t know. A properly formulated scientific hypothesis is falsifiable—there are steps we can take, at least in theory, to test the true state of the world, to determine if our hypothesis is true or not. In practice, this means considering alternative explanations ahead of time, before conducting the experiment, and designing the experiment so that the alternatives are ruled out.

If you’re trying out a new medicine on two groups of people, the experimental conditions have to be the same in order to conclude that medicine A is better than medicine B. If all the people in group A get to take their medicine in a windowed room with a nice view, and the people in group B have to take it in a smelly basement lab, you’ve got a confounding factor that doesn’t allow you to conclude the difference (if you find one) was due solely to the medication. The smelly basement problem is a known known. Whether medicine A works better than medicine B is a known unknown (it’s why we’re conducting the experiment). The unknown unknown here would be some other potentially confounding factor. Maybe people with high blood pressure respond better to medicine A in every case, and people with low blood pressure respond better to medicine B. Maybe family history matters. Maybe the time of day the medication is taken makes a difference. Once you identify a potential confounding factor, it very neatly moves from the category of unknown unknown to known unknown. Then we can modify the experiment, or do additional research that will help us to find out.

The trick to designing good experiments—or evaluating ones that have already been conducted—comes down to being able to generate alternative explanations. Uncovering unknown unknowns might be said to be the principal job of scientists. When experiments yield surprising results, we rejoice because this is a chance to learn something we didn’t know. The B-movie characterization of the scientist who clings to his pet theory to his last breath doesn’t apply to any scientist I know; real scientists know that they only learn when things don’t turn out the way they thought they would.

In a nutshell:

1. There are some things we know, such as the distance from the Earth to the sun. You may not be able to generate an answer without looking it up, but you are aware that the answer is known. This is Rummy’s known known.
2. There are some things that we don’t know, such as how neural firing leads to feelings of joy. We’re aware that we don’t know the answer to this. This is Rummy’s known unknown.
3. There are some things that we know, but we aren’t aware that we know them, or forget that we know them. What is your grandmother’s maiden name? Who sat next to you in third grade? If the right retrieval cues help you to recollect something, you find that you knew it, although you didn’t realize ahead of time that you did. Although Rumsfeld doesn’t mention them, this is an unknown known.
4. There are some things that we don’t know, and we’re not even aware we don’t know them. If you’ve bought a house, you’ve probably hired various inspectors to report on the condition of the roof, the foundation, and the existence of termites or other wood-destroying organisms. If you had never heard of radon, and your real estate agent was more interested in closing the deal than protecting your family’s health, you wouldn’t think to test for it. But many homes do have high levels of radon, a known carcinogen. This would count as an unknown unknown (although, having read this paragraph, it is no longer one). Note that whether you’re aware or unaware of an unknown depends on your expertise and experience. A pest-control inspector would tell you that he is only reporting on what’s visible—it is known to him that there might be hidden damage to your house, in areas he was unable to access. The nature and extent of this damage, if any, is unknown to him, but he’s aware that it might be there (a known unknown). If you blindly accept his report and assume it is complete, then you’re unaware that additional damage could exist (an unknown unknown).

We can clarify Secretary Rumsfeld’s four possibilities with a fourfold table:

What we know that we know: GOOD—PUT IT IN THE BANK	What we know that we don’t know: NOT BAD, WE CAN LEARN IT
What we don’t know that we know: A BONUS	What we don’t know that we don’t know: DANGER—HIDDEN SHOALS

The unknown unknowns are the most dangerous. Some of the biggest human-caused disasters can be traced to these. When bridges collapse, countries lose wars, or home purchasers face foreclosure, it’s often because someone didn’t allow for the possibility that they don’t know everything, and they proceeded along blindly thinking that every contingency had been accounted for. One of the main purposes of training someone for a PhD, a law or medical degree, an MBA, or military leadership is to teach them to identify and think systematically about what they don’t know, to turn unknown unknowns into known unknowns.

A final class that Secretary Rumsfeld didn’t talk about either are incorrect knowns—things that we think are so, but aren’t. Believing false claims falls into this category. One of the biggest causes of bad, even fatal, outcomes is belief in things that are untrue.