Quotes About Base rate

For example, if you believe that 3% of graduate students are enrolled in computer science (the base rate), and you also believe that the description of Tom W is 4 times more likely for a graduate student in that field than in other fields, then Bayes's rule says you must believe that the probability that Tom W is a computer scientist is now 11%. If the base rate had been 80%, the new degree of belief would be 94.1%.

There is one thing you can do when you have doubts about the quality of the evidence: let your judgments of probability stay close to the base rate. Don't expect this exercise of discipline to be easy—it requires a significant effort of self-monitoring and self-control.

The essential keys to disciplined Bayesian reasoning can be simply summarized: Anchor your judgment of the probability of an outcome on a plausible base rate. Question the diagnosticity of your evidence.

The importance of the base rate was made famous by the psychologist Daniel Kahneman, who coined the phrase "the outside view and the inside view." The inside view means looking at the specific case in front of you: this couple. The outside view requires you to look at a more general "comparison class" of cases—here, the comparison class is all married couples. (The outside view needn't be statistical, but it often will be.)

The key to statistical prediction is to figure out how much weight you should assign to the base rate and specific case. If the expected accuracy of the prediction is low, you should place most of the weight on the base rate. If the expected accuracy is high, you can rely more on the specific case.