The ETTO Principle

Erik Hollnagel

Ph.D., Professor, Professor Emeritus


Hollnagel, E. (2009). The ETTO Principle: Why things that go right sometimes go wrong. Farnham, UK: Ashgate.


What is arguably one of the most influential papers in contemporary psychology starts rather tantalisingly as follows:

My problem is that I have been persecuted by an integer. For seven years this number has followed me around, has intruded in my most private data, and has assaulted me from the pages of our most public journals. ... The persistence with which this number plagues me is far more than a random accident. ... Either there really is something unusual about the number or else I am suffering from delusions of persecution.

(The paper in question is George Miller’s ‘The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information’ published in 1956. This paper introduced to the general public the notion of limitations in human short-term memory and attention, and proposed as quantification an integer that since then

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