Survivorship Bias: The Trap of Seeing Only What Remains
"The S&P 500 has always eventually recovered." Is that the whole truth? The companies that failed to recover and were dropped from the index are not part of this story.
What Is Survivorship Bias?
Survivorship bias is a cognitive error that distorts reality because only the survivors are visible.
During World War II, there was a plan to analyze bullet holes in returning bombers and reinforce those areas. But the statistician Abraham Wald argued the opposite: "Where were the planes that did not return hit?" His point was that you should reinforce the areas that were hit on the planes that did not return.
Investing is the same. The successful companies, successful funds, and successful strategies we see hide countless cases of failure.
The S&P 500 and Survivorship Bias
The S&P 500 is composed of the top 500 U.S. companies. What matters is that this composition keeps changing.
There are companies that grow and get added to the index, and companies that decline or go bankrupt and are dropped. Fewer than half of the companies in the S&P 500 in 1980 were still in it in 2020. Kodak, Sears, GE (whose weight fell dramatically), Blockbuster... all were once core companies of the S&P 500.
The statement "the S&P 500 has always recovered" is a result made possible in part because the index's own composition keeps being replaced, favoring the companies that survived.
Investing in an individual stock is different. Someone who invested in AOL in 2000 was still at a loss in 2020.
Survivorship Bias in Funds and ETFs
Survivorship bias is rampant in comparisons of active-fund performance too.
Studies show that among active funds surviving 15-20 years or more, only 10-20% beat the market average (an index). And even this is a statistic of surviving funds only. Including funds that were shut down due to poor performance, the share that beat the market drops even lower.
"That fund manager beat the market 10 years in a row" -> but you also need to look at how the other funds that manager ran turned out.
This Service and Survivorship Bias
This service is not completely free of survivorship bias either.
Existing indexes (the S&P 500, KOSPI, etc.) are already markets that survived. In the early 20th century, the Argentine, Russian, and German stock markets were effectively wiped out by war or revolution. The proposition that "stocks rise over the long term" is based on the developed markets that survived.
For single-stock data, only currently listed companies can be analyzed. Delisted companies are invisible in the data. Interpret results with this in mind.
Preguntas frecuentes
Q. Is survivorship bias a reason to reject index investing?
No. Index investing is actually one way to respond to survivorship bias. Because you buy the whole index rather than picking individual companies, you do not need to predict which companies will thrive and which will fail. However, the assumption that "this entire market will survive" is still required.
Q. How does survivorship bias affect this calculator's data?
When you calculate based on an index (ETF), the index's own composition changes are already reflected. Individual-stock calculations are limited to currently listed companies. There is no data for companies with a delisting history. It is important to interpret results with awareness of these limits.
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📋 Los resultados se basan en datos históricos; las rentabilidades pasadas no garantizan rentabilidades futuras.
📋 Este servicio se ofrece con fines educativos para ayudarte a entender la inversión, no como asesoramiento de inversión.