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Tied upper ranks on a finite multiset

Statement

Let a1,…,am∈R with m≥1. Let J be uniform on {1,…,m} and let U be independent of J and uniform on [0,1]. Set R0:=#{i:ai≥aJ}m,RU:=#{i:ai>aJ}+U#{i:ai=aJ}m. Then RU is uniform on [0,1], while R0 is superuniform: P(R0≤u)≤u for every u∈[0,1].

Facts & Assumptions

Given: A nonempty finite list of real numbers, a uniform random index J, and an independent uniform variable U.

[F1]

A uniform law on a nonempty finite set assigns equal weight 1/m to each point. The uniform probability space on a nonempty finite set

[F2]

Every nonempty finite list of real values has a maximum; repeatedly removing all indices at that maximum sorts the distinct values. Every nonempty finite set of reals has a maximum and a minimum

[F3]

The tie breaker is independent of the selected index. Independent random elements

[A1]

U has the uniform Borel law on [0,1], so its probability on an interval is that interval's length.

Proof

1.1F1F2F3A1givenconstruct

Starting from the given list, repeatedly take the maximum of the remaining entries and remove every index carrying that value. By [F2] each nonempty remaining list has a maximum, and each iteration removes at least one of the original m indices, so the process ends after at most m iterations. It produces distinct values b1>b2>⋯>bk with block sizes rj. Put cj:=∑ℓ<jrℓ, so ∑jrj=m. By [F1], the event that aJ=bj has probability rj/m. On that event, R0=(cj+rj)/m and, by [A1] and [F3], RU=(cj+rjU)/m, uniform on the interval Ij=[cj/m,(cj+rj)/m] of length rj/m.

2.1A1step 1.1algebra

By step 1.1, the intervals Ij are consecutive, begin at 0, and end at 1. Conditional on aJ=bj, RU=(cj+rjU)/m, so for u∈[0,1] its conditional CDF is 0 below the left endpoint, (mu−cj)/rj between the endpoints, and 1 above the right endpoint, by [A1]. Multiplying each conditional CDF contribution by P(aJ=bj)=rj/m gives the length of Ij∩[0,u]. These lengths sum to u because the intervals partition [0,1] up to shared endpoints. Hence P(RU≤u)=u for every u∈[0,1], so RU is uniform.

3.1step 1.1step 2.1algebra∎

For u∈[0,1], P(R0≤u) is the sum of rj/m over those blocks whose right endpoint (cj+rj)/m is at most u. These blocks form an initial segment; if the last is j, the sum telescopes to (cj+rj)/m≤u, and if there is no such block the sum is zero. Hence P(R0≤u)≤u.

Depends on

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Sources