“We’re really bad with numbers”

We have a tendency to base judgments on known specific numbers and percentages, ignoring necessary general statistical information. This way we often erroneously over evaluate options with high numbers and percentages, ignoring what subset or base these numbers apply to…

Base Rate Neglect

Base Rate Neglect

Scientific research example:

Imagine you’re the major of a city with 1 million inhabitants, with 100 known criminals. Your citizens want you to decrease crime rate. Your Police Chief suggests installing a surveillance camera with automatic facial recognition software. The software has a failure rate of only 1%.

Is installing the camera a good idea? Most of the people think so.
However… Imagine that the whole city passes in front of the camera. You will catch about 99 of the 100 terrorists. But, you will also catch about 9,999 innocent citizens…

Online Persuasion tips:

When mentioning numbers or percentages:


  • Supersize your numbers and percentages by changing ‘the base’ (e.g. 99% of our active clients give us a 5-star rating, instead of 80% of all our clients).
  • Do the opposite for negative numbers (0% of our active users is unsatisfied with our product, instead of 20% of our users is unsatisfied).


Further reading on base rate neglect and base rate fallacy:

  • Base rate fallacy on Wikipedia
  • Amos Tversky & Daniel Kahneman, “Evidential Impact of Base Rates”, in Judgment Under Uncertainty: Heuristics and Biases, Kahneman, Paul Slovic, and Tversky, editors (1985), pp. 153-160.