Base Rate Neglect
How trustworthy is the conviction 'this startup will definitely hit it big'? The moment you forget the 'overall probability' that most startups fail, your judgment begins to go badly astray.
What Base Rate Neglect Is
The base rate is the 'overall baseline probability' of how often something generally happens — for example, 'the success rate of startups' or 'the probability that an active fund beats the index.'
Base-rate neglect is the cognitive bias of forgetting or taking lightly this overall probability, and overreacting to concrete, vivid individual information right in front of you. It's a representative bias revealed in the research of psychologists Daniel Kahneman and Amos Tversky.
In their 1973 experiment, people were captivated by a personality description (concrete information) and almost entirely ignored the group composition ratio (the base rate) of 'which group he was drawn from.' A vivid story beat a bland statistic.
Base rate neglect is connected to the 'representativeness heuristic.' We tend to judge probability by 'how much it resembles a typical image' rather than 'how common it is.'
The Illusion in Numbers: A Medical-Test Example
There's a classic example. Suppose there's a disease that 1% of the population has, and a test that detects it with 90% accuracy. If your test comes back positive, what's the probability you actually have the disease?
Many people — even doctors — answer '90%.' But the correct answer is only about 8%.
Here's why. Because the disease is very rare (base rate 1%), the number of 'false positives' arising among the healthy majority is far greater than the number of 'true positives' among actual patients. So even a positive result is mostly a false alarm from a healthy person. Ignore the base rate (1%) and focus only on the test accuracy (90%) right in front of you, and you'll misjudge the probability by more than tenfold.
The key point of this example is that 'the rarer the event, the more the base rate dominates the result.' No matter how accurate a signal looks, if the event is rare to begin with, false alarms can overwhelm the true ones.
Base Rates Easily Forgotten in Investing
Base rate neglect happens often in investing too.
First, jackpot hopes for startups and individual stocks. The judgment 'this company has a good product and a capable CEO, so it'll hit big' is vivid, but it erases the base rate that 'the vast majority of new companies fail within a few years.'
Second, active-fund selection. The expectation 'this fund manager is brilliant, so they'll beat the market' ignores the base rate that few active funds beat the market over the long run. According to S&P's SPIVA data as of the end of 2024, over 15 years about 89.5% of U.S. large-cap active funds failed to beat the S&P 500 index. Put another way, pick one at random and the base probability of beating the index is only around 10%.
Before being drawn to a vivid success story, first asking 'how often does this sort of thing actually happen?' is the first step to preventing base rate neglect.
The active-fund underperformance statistic is not to condemn a particular fund, but teaching material showing how far the expectation 'this fund will win' differs from the base rate.
常见问题
Q. What on earth is a base rate?
The base rate is the 'overall baseline probability' of how often something generally happens. For example, 'the success rate of startups,' 'the prevalence of a disease,' and 'the probability that an active fund beats the index' are all base rates. When judging an individual case, you must also consider this baseline probability for your judgment not to go badly astray.
Q. Are you saying to trust statistics over vivid stories?
You should look at both, but the point is to beware of forgetting the base rate entirely because you're captivated by vivid individual information. A concrete success story leaves a strong impression, but you can only gauge probability properly if you also weigh 'how common this sort of thing is to begin with.'
Q. Is there a simple way to reduce base rate neglect?
It's effective to build the habit of first asking, before you judge, 'how often does this usually happen?' Recalling the overall probability before evaluating the appeal of an individual case can, to some degree, keep you from being pulled too far by the story in front of you.
📋 结果基于历史数据计算,过去的收益不代表未来的收益。
📋 本服务旨在帮助理解投资、供教育之用,并非投资建议。