The Dangers of Survivorship Bias
Today at a Glance
- In a famous story, Cicero wrote of Diagoras, an atheist who, when confronted with paintings of people who prayed and then were saved from a shipwreck, replied, "I see those who were saved, but where are those painted who prayed and drowned?"
- Survivorship Bias is the error resulting from systematically focusing on survivors (successes) and ignoring casualties (failures) that causes us to miss the true base rates of survival (the actual probability of success) and arrive at flawed conclusions.
- To avoid the trap, we must consider the unseen evidence just as much as the seen. To develop a better perspective on base rates of success, consult a Possibility Grid, which lays out completed and not completed actions of both winners and losers.
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2,000 years ago, the Roman philosopher and politician, Cicero, told the story of an atheist named Diagoras, who had been vocal in his non-belief about the gods.
Those trying to convince Diagoras of the existence of higher powers showed him a series of painted tablets that portrayed a group of sailors who had prayed during a vicious storm and then survived the shipwreck.
Diagoras looked at the paintings and replied, "I see those who were saved, but where are those painted who prayed and drowned?"
Diagoras (through Cicero) highlighted an all-too-common flaw in human reasoning and decision-making:
We ground ourselves in the seen and systematically ignore the unseen.
We admire the "winners" and study their success stories in hopes of determining the 10 common traits that led to their success.
We applaud those who take risks and succeed, creating a mental model of risk and reward that fundamentally misses all those who took risks and failed.
We long for this simplicity, but the truth is that it misses a critical set of underlying realities that we all need to be aware of.
Let's talk about the dangers of Survivorship Bias...
The Statistician Saved Their Lives
A young statistician saved thousands of lives.
During World War II, the U.S. was facing issues with its bombers in the Pacific and decided to add reinforcement armor to specific areas of its planes.
A group of analysts examined returning bombers and plotted the bullet holes and damage on them.
Based on this analysis, they came to the conclusion that adding armor to the tail, body, and wings would improve their odds of survival.
But a young statistician named Abraham Wald immediately noted that this would be a tragic mistake...
By only plotting data on the planes that returned, they were systematically omitting the data on a critical, informative subset: The planes that were damaged and unable to return.
Abraham Wald had recognized a simple fact:
- "Seen" planes had sustained damage that was survivable.
- "Unseen" planes had sustained damage that was not survivable.
Wald concluded that, contrary to the original analysis, armor should be added to the unharmed regions of the returning planes. Where the survivors were unharmed was actually where the planes were most vulnerable.
Based on his insight, the military reinforced the engine and other vulnerable parts, significantly improving the safety of the crews during combat and saving thousands of lives.
Avoiding Survivorship Bias
Abraham Wald had identified a cognitive bias called Survivorship Bias:
The error resulting from systematically focusing on survivors (successes) and ignoring casualties (failures) that causes us to miss the true base rates of survival (the actual probability of success) and arrive at flawed conclusions.
We can see examples of Survivorship Bias all around us:
- We read books on the common traits of successful people, but fail to consider all of the unsuccessful people who possessed those same traits.
- We applaud the belief when we hear that an entrepreneur took out a second mortgage and succeeded, but fail to consider all of the entrepreneurs who did the same and went bankrupt.
- We celebrate the "bet on yourself!" success stories but ignore the causalities of the same mantra.
- We study the cultural strategies of the most successful companies, but fail to consider all of the companies that followed those same strategies and fell apart.
When we fail to consider the range of outcomes and the hidden evidence, we develop a skewed (and often incorrect) view of reality—particularly of the risks and the true rates of success.
To avoid the trap, when evaluating evidence and making a decision, consider all four quadrants of this simple 2x2 Possibility Grid:
- Q1: Completed Action & Won
- Q2: Completed Action & Lost
- Q3: Did Not Complete Action & Won
- Q4: Did Not Complete Action & Lost
The Possibility Grid forces you to consider and highlight the unseen just as much as the seen.
While there are certainly character traits and actions that improve the odds of financial, business, or personal success, it is altogether dangerous to assume that there is any blanket formula that is guaranteed to work.
Survivorship Bias cannot be avoided altogether, because the vast majority of books and history are written by and about the survivors and victors, but wherever possible, consult the 2x2 grid and consider the unseen evidence.
Remember: What is unseen often has just as much value as what is seen.