During World War II, fighter planes would come back from battle with bullet holes. The Allies initially sought to strengthen the most commonly damaged parts of the planes to increase combat survivability.
A mathematician, Abraham Wald, pointed out that perhaps the reason certain areas of the planes weren’t covered in bullet holes was that planes that were shot in certain critical areas did not return. This insight led to the armour being reinforced on the parts of returning planes where there were no bullet holes. This wisdom was also beneficially applied to the Skyraider during the Korean War.
This shows that sometimes the missing data may be more meaningful than the available data. This phenomena is now known as “survivorship bias“. Is not only true for matters of aircraft design and defence during war time. We shouldn’t only assess the evidence available to us but consider the evidence we don’t have, because it might have been shot down.