Johari Window
Known Unknowns Vs. Unknown Unknowns
The categorization of knowledge about a topic where 'known unknowns' are risks that one is aware of, while 'unknown unknowns' are risks that come from situations that are so unexpected that they would not be considered.
EverydayConcepts.io
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Origin
Named for psychologists Joseph Luft and Harrington Ingham who devised the broader concept into a 2x2 matrix in 1955 as a self-help exercise. Famously used in 2002 to articulate military intervention by the US into Iraq due to 'unknown' existence of weapons of mass destruction.