When a vivid case drives certainty, ask for the base rate first
When a vivid individual case makes you feel certain about probability, explicitly ask 'what is the actual frequency of this event in the relevant population?' before forming any judgment.
Why This Is a Rule
A single vivid case hijacks your probability estimates. Your friend's startup succeeded spectacularly, so startups feel like good bets. A colleague's child had a vaccine reaction, so vaccines feel dangerous. One dramatic product failure makes an entire vendor feel unreliable. In each case, a single vivid example overwrites the actual frequency distribution in your mind.
This is the representativeness heuristic: your brain substitutes "how well does this case match the category?" for "how frequent is this outcome in the population?" The vivid case matches the category perfectly — it is the category in your mind — so you judge the probability as high. Meanwhile, the base rate (actual frequency across all cases) might tell a completely different story.
The corrective is simple but must be explicit: when a vivid case is driving your sense of certainty, ask "what is the actual frequency of this event in the relevant population?" This question forces you to shift from case-based reasoning (representative but misleading) to population-based reasoning (boring but calibrated).
When This Fires
- A dramatic example is shaping your risk assessment (startup success, medical risk, system failure)
- You feel very certain about a probability based on one or two memorable cases
- Someone is persuading you with a compelling anecdote rather than data
- You're about to make a decision and your confidence comes from a story, not statistics
Common Failure Mode
Acknowledging the base rate but letting the vivid case override it anyway. "I know most startups fail, but this one is different." The base rate becomes a disclaimer rather than an anchor. Your judgment still comes from the vivid case — you've just added a caveat to feel rational about an irrational estimate.
The Protocol
When a vivid case triggers certainty: (1) Name the case explicitly: "I feel certain because of [specific example]." (2) Ask: "What is the actual frequency of this outcome in the relevant population?" (3) Look up or estimate the base rate. (4) Start your judgment from the base rate, then adjust for case-specific factors — not the other way around. The base rate is the anchor; the vivid case is the adjustment.