During world war 2, the Allies were losing too many of their aeroplanes to enemy fire. So, they decided to strengthen the aeroplanes by adding additional protective and reinforcing armour. However, adding too much armour would make the planes too heavy to fly. So it was decided that only those areas of the aeroplane which received maximum enemy fire were to be armoured. An analysis of all those aeroplanes which had returned safely gave the following picture:

The red dots indicated the areas which had received maximum enemy fire. Hence, they decided to add extra armour at these locations to strengthen the aeroplanes against enemy fire. But they were made to change their logic by a mathematician named Abraham Wald. Mathematicians always take great pleasure in proving common logic to be wrong.
Abraham proposed that those areas of the aircraft which had not received any enemy fire should be armoured, leaving out the red dot areas. His logic: These are the aircraft that survived the enemy fire and returned safely, despite getting hit by enemy fire. The ones that did not return were most possibly those which probably got hit in those parts that are more critical and not indicated here by the red dots.

The judgemental error that was happening here was that of focusing on the aeroplanes that survived and neglecting those that perished. Our tendency to focus on the winners and create a winning formula from their success story while disregarding all those who failed in the same process is called survivorship bias.
One of the reasons we tend to fall into the survivorship bias is that there is a lot of data available about the winners (the planes that came back safely) while there is not much data or nobody is interested in documenting the data of the failures and also-rans (the aeroplanes which did not come back). We are just not interested in looking at the loser’s data. This approach is usually disastrous.
In the next post, we will see a striking example of how survivorship bias emerges and thrives. If you are keen on reading it now itself, just click the link below: https://vishwajeetsinsights.in/survivorship-bias-part-2-3-survivorship-bias-in-the-game-of-coin-tossing/