Statistics for Traders #47: The Improved Rule of Three 3/(n+1) — the Taylor-linearized Bayesian Beta(1,1) uniform-prior 95% upper bound on p when x=0 in n trials, and a uniformly better approximation to the exact 1 - α^(1/n) than the plain 3/n. Applied to today's slot 5 AUDUSD × Australia GDP big_miss 0-of-3 UP at 15m: Rule of Three gives 100% (uninformative); Improved gives 75%; exact CP gives 63.16%; Beta(1,1) posterior gives 52.71% — none reject fair-coin at n=3. Only n=4 unanimity is enough for the Beta(1,1) lower bound to clear 50%. Same-day pairing with today's PT #47 Jovanovic-Levy 1997 primary source.
The improved Rule of Three is 3/(n+1) — Jovanovic and Levy (1997) showed it is both a uniformly better approximation to the exact 95% upper bound 1 − α^(1/n) than the plain 3/n, and the Taylor- linearization of the Bayesian upper credibility bound 1 − α^(1/(n+1)) under a Beta(1,1) uniform prior on p. Applied to today’s slot 5 AUDUSD × Australia GDP big_miss 0-of-3 UP at 15m: 3/n = 100% (uninformative), 3/(n+1) = 75%, exact CP = 63.16%, Beta(1,1) = 52.71%. None of the four rejects fair-coin at n=3. Same-day pairing with today’s PT #47 Jovanovic-Levy 1997 primary source.

Jovanovic-Levy 1997 Table 1 reproduction
| n | 3/n | 3/(n+1) | Exact 1−α^(1/n) | Beta(1,1) 1−α^(1/(n+1)) |
|---|---|---|---|---|
| 3 | 1.00000 | 0.75000 | 0.63160 | 0.52713 |
| 4 | 0.75000 | 0.60000 | 0.52713 | 0.45072 |
| 5 | 0.60000 | 0.50000 | 0.45072 | 0.39304 |
| 10 | 0.30000 | 0.27273 | 0.25887 | 0.23840 |
| 20 | 0.15000 | 0.14286 | 0.13911 | 0.13295 |
| 50 | 0.06000 | 0.05882 | 0.05816 | 0.05705 |
| 100 | 0.03000 | 0.02970 | 0.02951 | 0.02923 |
Ordering holds for every n: 3/n ≥ 3/(n+1) ≥ exact ≥ Beta(1,1). At n=3 the four bounds span 100.0% to 52.7% — a 47-point spread. By n=100 all four agree within 0.08 pp. The practical upshot: for n < 20, method choice matters more than method precision.
Applied to today’s slot samples
| Sample | 3/n | 3/(n+1) | Exact CP | Beta(1,1) | Reject 50%? |
|---|---|---|---|---|---|
| AUDUSD AU GDP big_miss 0/3 UP (15m) | 100.00% | 75.00% | 63.16% | 52.71% | none |
| GBPNZD UK Svcs big_beat 3/3 UP (5m-4h)* | 0.00% | 25.00% | 36.84% | 47.29% | none |
| AUDUSD AU GDP big_beat 4/4 UP (15m)* | 25.00% | 40.00% | 47.29% | 54.93% | Beta(1,1) only |
| Yesterday's GBPJPY big_miss 0/4 UP (5m) | 75.00% | 60.00% | 52.71% | 45.07% | Beta(1,1) only |
* On unanimity samples x = n, the reported bound is the LOWER 95% bound on the up-rate (i.e. 1 minus the upper bound on the down-rate).
n=3 unanimity is not enough — even the Beta(1,1) 95% posterior lower bound stays under 50%. n=4 unanimity IS enough for Beta(1,1) (54.93% > 50%) but not for exact Clopper-Pearson (47.29%). The method choice flips the decision.
Verification note
All four columns of Table 1 verified 2026-09-16 via Python (1 - 0.05**(1/n), 3/n, 1 - 0.05**(1/(n+1)), 3/(n+1)) — five- decimal match with the Jovanovic-Levy 1997 published table across every row. Same-day pairing with today’s Paper Trail #47. Chart built via a one-off script reusing scripts/insights-charts/svg.ts + theme.ts + sharp, not committed under scripts/.