You assess frequency and probability by ease of recall. If examples come to mind quickly, the event feels common; if recall is hard, it feels rare.
This is availability. It is often useful because memory tracks experience. But memory is also biased by salience, recency, and vividness.
A dramatic story can outweigh quiet statistics. A recent incident can dominate your sense of risk. What is easy to imagine becomes easy to believe.
The fast system turns fluency into judgment: “I can think of many cases, therefore it must be frequent.” The slow system can check with data, but it often does not.
Availability explains why public fears detach from actual rates, and why personal experience can masquerade as universal evidence. Ease is not frequency.