Check whether you are estimating frequency or vividness

Why This Is a Rule

The availability heuristic makes you substitute "how easily can I picture this?" for "how often does this actually happen?" Plane crashes are vivid and memorable, so you overestimate their frequency. Slow-developing diseases are invisible and abstract, so you underestimate theirs. Your brain doesn't distinguish between "easy to recall" and "common" — it treats one as evidence for the other.

This rule inserts a diagnostic question at the exact moment the substitution happens: when you're about to estimate a frequency or probability. "Am I estimating actual frequency, or how easily I can picture this?" If the answer is vividness, you've caught the bias before it corrupts your decision. The corrective action — looking up the base rate — replaces the heuristic with data.

When This Fires

• Estimating project failure probability ("most projects like this fail")
• Assessing risk of rare events (security breaches, market crashes, illness)
• Deciding how much to worry about edge cases versus common cases
• Any time you say "this happens all the time" or "this almost never happens" without data

Common Failure Mode

Trusting your frequency estimate because you can recall multiple examples. Recalling three vivid examples of a project failing feels like strong evidence for a high failure rate — but your memory is biased toward dramatic, unusual, and recent events. The hundreds of projects that succeeded quietly left no memorable trace. Your sample is biased toward what's vivid, not what's frequent.

The Protocol

When you catch yourself estimating a probability: (1) Pause and ask the diagnostic question. (2) If your estimate is based on how easily you can picture the outcome, flag it as availability-biased. (3) Look up the actual base rate — how often does this class of event actually occur? (4) Anchor your decision to the base rate, adjusting only for factors specific to your situation that the base rate doesn't capture.