Unknown unknown

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An unknown unknown, sometimes shortened to unk-unk, typically refers to an uncertainty that was unanticipated and, hence, unaccounted for in a formal model (eg, in a decision basis). Unknown unknowns can also refer to unanticipated decision variables, although this usage is much less common.

US Secretary of Defense Donald Rumsfeld gave the following helpful definitions at the Defense Department Briefing on 12 February 2002:

“Reports that say that something hasn’t happened are always interesting to me, because 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.”

This statement somewhat unfairly won Donald Rumsfeld the 2003 Foot in Mouth Award from the Plain English Campaign.