“You Can’t See Me “— The Survivorship Bias

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Survivorship bias or survival bias is a logical error where we concentrate on people or things that made past some sort of selection over time ignoring people or things that didn’t. It is a cognitive error i.e. it is a result of flawed thinking & is universal in nature. It affects our decisions all the time.

One of the most remarkable examples of survivorship bias happened during World War II. Abraham Wald, a Jewish Hungarian mathematician, Wald was a member of the Statistical Research Group (SRG) at Columbia University, where he applied his statistical skills to various wartime problems. One of the problems they were working on was how to reduce aircraft casualties. They studied bullet holes in returning aircraft and found that most of the bullet holes were around the wings and tail of the aircraft, with very few holes around the engine. US military’s conclusion was simple — The enemy was attacking the wings and tail of the aircraft & hence those areas should be fortified to prevent damage.

Wald stepped in and realized that Aircrafts which were attacked on areas near their engines did not return at all. Only the aircraft that avoided damage to its engine was able to return. Only the survivors returned. Hence, the engine is the most critical part of the aircraft which needs fortification and not the wings or tail, as aircraft could take a lot of bullets in those areas & still return safely. Wald’s observation led to significant improvement in casualty numbers and definitely helped in winning the war.

Anytime we are studying only the successful or the ones who are present now, it is a perfect example of survivorship bias. One idolizes movie stars, ignoring vast numbers which never tasted success. One follows college dropouts who created multi-billion dollar businesses, ignoring millions of dropouts who struggled in their life or whose businesses didn’t do well. One follows the success mantras of startups that do well, ignoring the fact that 95% of startups fail.

Beatles were initially rejected by record labels and was told: “guitar groups are on the way out”. They nevertheless became the most successful band in history. Examples like these are often cited to motivate people or I should rather say put forth half-baked notions. If only a small percentage of the population is ‘successful’, in what we want to achieve, we want to be like them & believe anything. We do not consider extraordinary risks, talent, circumstances, support, and sheer luck which made them what they are. We ignore that many others with similar talents and skills certainly didn’t have the same outcome. We see what is visible, ignoring what we can’t see.

Next time you hear a success story, think about how many people failed at doing the same thing. It is important to fully understand the risks and efforts that you need to put in. If you only look at survivors, you fail to learn about what didn’t work & what all could go wrong.

When you ask people going to a gym about how important their health is. Chances are the vast majority will say that it is very important. Is it then correct to conclude that “Health-conscious individuals are more likely to go to a gym”? Do you remember similar headlines in newspapers or advertisements from companies with statements like “9/10 of our customers recommend our service”? Survivorship bias is at play.

One has to look at all the Health-conscious individuals in a population & check if they are going to the gym or not — not the other way around. Maybe out of all the health-conscious individuals only 10% went to the gym, 50% preferred cycling, 35% preferred running and 5% did nothing. Similarly, Customers of a company would anyway prefer that company since they are already their customers. Who will be a customer if they are not happy with their service?

To avoid survivorship bias, it is hence important to look at all the things that started on the same path & look at each outcome. You will never know if you succumb to survivorship bias. Your decisions should be based on research, logic, and understanding of the entire population and not only those that survived. As Daniel Kahneman puts it, find good reasons for your decisions.

We think, each of us, that we’re much more rational than we are. And we think that we make our decisions because we have good reasons to make them. Even when it’s the other way around. We believe in the reasons, because we’ve already made the decision. — Daniel Kahneman