Base Rates
A base rate is the general frequency of an outcome in a population before you look at any specific case. If 90% of restaurants fail within five years, that's the base rate — and any new restaurant's plan needs to overcome it, not ignore it.
Why Base Rates Matter
People naturally focus on the specific details of a situation and neglect the background frequency. A startup founder hears her own compelling story; she doesn't hear the thousands of equally compelling stories that ended in failure. This is sometimes called "base rate neglect" and is one of the most common Cognitive Biases.
Probabilistic Thinking starts with base rates. Before asking "will this specific thing work?", ask "how often do things like this work in general?" Then adjust from there as you gather evidence specific to your case.
Connections
Anchoring Bias is partly a base rate problem — when an irrelevant number anchors your estimate, you drift away from the true base rate. Razors work precisely because they align with base rates: most explanations really are simpler than they appear, most people really aren't acting with malice.
Inversion provides a complementary approach: instead of asking "why will I beat the base rate?", ask "what specific advantages do I have that the typical case does not?"