Steelman
Base rate fallacy
Also called base rate neglect, neglecting prior probability.
Ignoring how common something is in the first place.
What it is
Judging a probability from specific information, such as a test result or a description, while ignoring the base rate (how common the thing is in the population), when both are relevant.
The pattern
The test is 99% accurate and came back positive. Therefore there’s a 99% chance I have the disease. (Ignoring that the disease is rare.)
An example
A disease affects 1 in 1,000 people. A test catches 99% of cases but also gives a false positive for 5% of healthy people. You test positive, so you conclude you almost certainly have the disease.
Picture 100,000 people: 100 are sick and 99 of them test positive; 99,900 are healthy and about 4,995 of them test positive. Of the 5,094 positives, only 99 are sick, which is about 2%.
Why it fails
When something is rare, even an accurate test produces more false positives from the huge healthy population than true positives from the tiny sick one. The base rate and the specific evidence both have to be combined.
When it’s not a fallacy
Most lists skip this part. Calling a fair point a fallacy is a mistake too.
It can be fair when
- The population base rate genuinely doesn’t apply. Someone with symptoms who is referred for a test isn’t a random member of the public, so their starting probability is higher.
Try one
A question from the app. The wrong answers are its usual look-alikes.
Parents’ group chat. Dana: “The school’s wellness survey is out: 7 in 10 kids who said they feel depressed are on their phones five hours a day or more.” Mark: “Same survey: about 1 in 10 kids said they feel that way, and half the school is on their phones that much.” Dana: “Eli’s on his six hours a day. Odds are he’s one of the depressed ones. The phone goes in a drawer tonight.”
What’s the flaw?
Depression is far rarer than heavy phone use: half the school is on five hours a day, but only about 1 in 10 kids said they feel depressed. Run the numbers and a heavy user has about a 1-in-7 chance of being one of them, not “odds are.”
A fair reply
“Half the school is on that much and most of them aren’t depressed, so let’s talk to Eli before we read his screen time as a diagnosis.”
Often confused with
- Prosecutor’s fallacy
The prosecutor’s fallacy is base-rate neglect in court, confusing the chance of the evidence given innocence with the chance of innocence given the evidence.
- Conjunction fallacy
The conjunction fallacy overrates a detailed scenario. The base rate fallacy ignores how common the basic scenario is.
Where the name comes from
The base rate is how common something is to begin with, before you learn anything about the case in front of you. Think of it as the baseline every new clue starts from. “Base rate neglect” says the fallacy outright: that starting point gets ignored. “Prior probability” means the same thing: the chance before the new evidence comes in.
Sources
- The Base Rate Fallacy, The Fallacy Files
In the app
Practice spotting it on arguments that sound real.
Steelman has more examples of base rate fallacy, how to answer it in a real conversation, and practice rounds that mix it with its look-alikes, so you learn the difference, not just the name. Coming soon to iPhone and Android.
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