While the constant comparison of notes can help us to detect differences, no technique can help us to see what is missing. But we can make it a habit to always ask what is not in the picture, but could be relevant. This…does not come naturally to us.

One of the most famous figures to illustrate this skill is the mathematician Abraham Wald (Mangel and Samaniego 1984). During World War II, he was asked to help the Royal Air Force find the areas on their planes that were most often hit by bullets so they could cover them with more armour. But instead of counting the bullet holes on the returned planes, he recommended armouring the spots where none of the planes had taken any hits. The RAF forgot to take into account what was not there to see: All the planes didn’t make it back.

Sönke Ahrens, How to Take Smart Notes, p. 126

Sometimes the key to insight and innovation isn’t what you see but what you don’t see.

Notice what is missing.

Ask yourself, “What’s not in the picture?”