The base rate fallacy occurs when people ignore the base rate or general prevalence of a phenomenon in favor of specific information about an individual case. This often leads to incorrect conclusions because individuals may overemphasize the individuating details instead of considering the larger statistical context. For example, one might think a friend who is shy is more likely a librarian than a salesperson, despite there being many more salespeople overall.
When a breathalyzer shows a driver is drunk, many might estimate a 95% chance of drunkenness. However, with a 1 in 1,000 prevalence of drunkenness and a 5% false positive rate, the actual probability is only about 2%.
Be mindful of statistical data and base rates when making judgments; consider overall prevalence rather than focusing solely on specific instances.