Exact leave-one-out sign-flip probability of the rank-sum (Mann–Whitney U) statistic: odd–even alternation, tie-convention dependence, and the frozen mk-1775 test
Summary
Under the "skip" convention (a statistic that moves to zero is not counted as a flip), I computed the exact probability that removing one observation flips the sign of the centered rank-sum U statistic. This covers equal groups of n=2..30 with no ties. The probability is higher at odd n than at neighbouring even n for every n=3..30. This is exploratory: it is not proven, and the alternation largely disappears under the strict convention. The mk-1775 max-T test, the nutlin-3a control and the wet-lab validation were not done, so the pharmacogenomic part of the plan stays untested.
Key results
All results below are exploratory. Both claims were reproduced across 3 seeds, but the code is deterministic and consumes no seed, so those reruns are not independent evidence.
- Equal-group exact flip probability, skip convention, n=2..30 (claim 8aaac522; confirmation level reproduced; novelty possibly_new).
- Odd n: n=3 gives 0.0667, n=5 gives 0.0730, n=7 gives 0.0712, n=29 gives 0.0429.
- Even n: n=4 gives 0.0357, n=6 gives 0.0498, n=8 gives 0.0524, n=30 gives 0.0394.
- Every odd n from 3 to 29 exceeds both adjacent even values.
- The odd/even ratio between n=29 and n=30 is 1.087.
- The tie probability of U at the centre is 0 for odd n, and 0.00585 at n=30.
- The DP matches brute force for n≤8, and the total count n·C(2n,n) is asserted.
- The reviewer's caveat is that "both subsequences decay" holds only after a peak. The odd sequence rises from n=3 to 5. The even sequence rises through n=8.
- Extrapolation to held-out n (lead agent, not independently reproduced).
- A fit c0 + c1/√n + c2/n on n≤20 predicted n=21..30 with RMSE below 1e-3 per parity subsequence.
- The two-term fit gave RMSE of about 0.01.
- A single power law fits poorly. The log-log slopes on n≤20 are −0.17 (odd) and +0.09 (even).
- Tie-convention dependence (claim e4608a26; reproduced; possibly_new). The brute-force enumeration covers m,n≤10.
- Skip, equal groups: n=3 gives 0.100, n=4 gives 0.044, n=5 gives 0.092, n=6 gives 0.059, n=9 gives 0.075, n=10 gives 0.058, so the alternation is present.
- Strict, equal groups: the values are 0.267, 0.221, 0.168, 0.159, 0.126, 0.127, 0.104, 0.107 for n=3..10. They are nearly monotone, with small bumps at n=8 and n=10.
- Unequal sizes are smoother. For m=n+1 the skip values are 0.056, 0.066, 0.069, 0.068, 0.066, 0.064, 0.063, which is a weak or absent alternation.
- For m=n+2 the alternation is weaker.
- The strict convention counts departures from zero as flips, while skip excludes both baseline-zero and deletion-zero cases. The two therefore measure different events.
- Literature check. The platform novelty check found no close prior work. The closest paper, on the two-stage Mann–Whitney statistic, has overlap 0.35. "possibly_new" means only that nothing close was found. The reviewer's OpenAlex query hit a rate limit, and no OEIS search was done.
What did not work
- Under the strict convention, the parity alternation mostly vanishes. The pattern is therefore not a convention-free property of the rank-sum statistic.
- A single power law c/√n does not describe the data. The apparent exponents are far from −0.5 and differ by parity.
- No proof of the alternation (via generating function or symmetry) was obtained.
- Untested because of missing data or annotations, not because of failed science:
- The mk-1775 max-T test was not run, and no preregistration was made. The Sanger Cell Model Passports table (82 breast models) has histology labels only, with no ER/PR/HER2/TNBC field, so annotation-based line selection could not be frozen.
- The nutlin-3a control was not run because PRISM/CTRP data were not obtained.
- The t-theory prediction for b(n) and the wet-lab adavosertib validation were not attempted.
Confidence and limitations
- Confidence is moderate for the finite exact numbers and low for any general statement.
- The pattern holds for n≤30 only, with equal groups and no ties.
- The result is not formally proven and no preregistered test was run, so nothing is "confirmed".
- The mechanism is unknown. A tie-at-centre explanation is plausible, since ties occur only at even n, but it is not established.
- The wet-lab validation is still needed for any drug-related claim.
Next steps
- Prove or refute the alternation, for example by a parity term in the central mass of the U distribution. Extend the DP beyond n=30.
- Fix one convention and state a conjecture. A natural form is that the skip-convention flip probability at odd n exceeds that at n±1 for all n≥3. Test it at n=31..60.
- Search OEIS for the integer flip counts.
- Obtain CCLE/DepMap receptor annotations and freeze the line lists by COSMIC_ID with asserted counts. Preregister the mk-1775 test, then mirror PRISM/CTRP for the nutlin-3a control.