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Rule of succession and pricing first-time events

Some event contracts ask about things that have never happened: a first-ever ruling, a new kind of record, a debut technology milestone. With no base rate, readers often swing between treating the event as impossible and treating it as a coin flip. This SoftCta guide introduces two simple statistical tools — Laplace’s rule of succession and the rule of three — and explains when they help and when better knowledge should replace them.

Zero history is not zero chance

If something has not happened in the last ten years, that is evidence it is uncommon, not proof it cannot happen. Pricing an event at exactly zero is extreme overconfidence, discussed in our overprecision guide. The question is how small the probability is, and how fast new information should change it.

Laplace’s rule of succession

Pierre-Simon Laplace proposed a simple estimate: after s occurrences in n comparable trials, estimate the next-trial chance as (s + 1) divided by (n + 2). With zero occurrences in 18 years, that gives 1 in 20, or 5 percent per year. It is equivalent to a uniform prior updated with the data, linking it to Bayesian updating.

The rule of three

A related tool bounds rather than estimates. If an event has not occurred in n independent trials, an approximate 95 percent upper confidence bound for the per-trial probability is 3 divided by n. Zero occurrences in 30 comparable years suggests the annual chance is probably below about 10 percent. SoftCta readers use it as a sanity check on prices that look too high.

Choosing the trials

Both rules depend on what counts as a trial. Years, meetings, elections or launches each give different numbers. Using a narrow reference class gives few trials and wide uncertainty; a broad one may mix unlike cases. Our reference class guide and Fermi estimation guide help readers choose and decompose.

A worked example

A contract asks whether a first-ever event of a defined type will occur this year, priced at an illustrative 22 cents Yes. In 15 comparable years it has never happened. Laplace gives about 6 percent, and the rule of three suggests an upper bound near 20 percent. The reader then adds specific knowledge: a new policy makes the event more feasible this year. They write a range, convert with the implied probability tool, and verify live cents. Substantial risk of loss.

Limits and common mistakes

These rules assume trials are similar and independent; real-world conditions change. They ignore mechanism knowledge that may matter far more. They are crude with very few trials, as our small sample guide notes. Treating them as precise answers, or using them to justify sure-thing calls in either direction, misses the point.

Primary sources SoftCta readers open

SoftCta readers keep a simple worksheet: the defined event, the chosen trial unit, the count of trials and occurrences, the Laplace estimate, the rule-of-three bound, and notes on what specific evidence moves them away from those anchors. They keep commentary and social posts in a context column.

Reading the quote step by step

• Read the full title and rules, including the named settlement source and the time zone.
• Convert the Yes and No prices to implied probabilities and note the gap between them.
• Subtract fees and spread to see the effective price you would actually face.
• Write your own probability range and the evidence behind it, then compare.
• Check related and near-duplicate markets on other venues for consistency.

Closing SoftCta checklist

• Re-read the venue definition in your own words before reacting to a price.
• Convert prices with fee and spread awareness; verify live cents on the operator.
• Prefer primary settlement sources over secondary headlines and social summaries.
• Treat ChanceWire hubs, movers, and tools as orientation — not order entry.
• Confirm age, location, and product eligibility; SoftCta pages will not suggest location spoofing.
• If clarity is incomplete, watching is a complete SoftCta outcome — no pick-selling urgency required.

SoftCta habits

• Write your own probability range before checking the crowd price.
• Prefer dated primary sources over social summaries.
• Compare related markets when a single book looks extreme.
• Size curiosity with fees and depth in mind; substantial risk of loss.