ABC rules and the utilitarian–egalitarian spectrum¶
Advanced level (301). The formal treatment of approval-based committee (ABC) rules, following Lackner & Skowron's Multi-Winner Voting with Approval Preferences (§2.1–2.2). One running election pulls the major rules apart and lines them up on a single axis — from utilitarian (maximize total satisfaction) to egalitarian (cover as many voters as possible). Gentle version: the 101 intro. All committees below are verified with Lackner's own abcvoting.
The ABC-rule framework¶
An election instance is E = (A, k): an approval profile A (each voter i submits an approval set A(i) ⊆ C) and a target committee size k. An ABC rule maps E to one or more winning committees — size-k subsets of C.
- A rule is resolute if it always returns exactly one committee, irresolute if it may return several tied committees.
- Real systems force a single outcome with a tie-breaking order. Modelling all randomness as fixed before the election (a pre-published linear order over committees, "maximal committee wins") turns a randomised tiebreak into a deterministic one. This is precisely the "published lot" discipline STAR uses for its single-winner ties (see STAR tie-breaking) — same idea, lifted to committees.
The running instance (Example 2.1)¶
k = 4, C = {a,b,c,d,e,f,g}, 12 ballots:
3 × {a,b} 3 × {a,c} 2 × {a,d} 1 × {b,c,f} 1 × {e} 1 × {f} 1 × {g}
Approval counts: a=8, b=4, c=4, d=2, f=2, e=1, g=1.
Rule 1 — Approval Voting (AV): utilitarian¶
AV maximises the total number of approvals inside the committee. With score_AV(A,c) = |{i : c ∈ A(i)}| (how many voters approve c), AV picks committees W maximising score_AV(A,W) = Σ_{c∈W} score_AV(A,c) — i.e. the k most-approved candidates.
Top three are a,b,c; the fourth seat ties between d and f (both 2). So AV is irresolute here, returning two committees of equal AV-score 18: {a,b,c,d} and {a,b,c,f}. AV is the pure utilitarian rule — it maximises summed satisfaction and is indifferent to whether some voters are left with nothing. ({a,b,c,d} leaves 3 voters wholly unrepresented; {a,b,c,f} leaves 2 — same AV-score, so AV can't tell them apart.)
This av rule is exactly the LH engine's Approval_Multi_Winner (bloc Approval) — see the approval original.
Rule 2 — Approval Chamberlin–Courant (CC): egalitarian¶
CC is, in the book's words, "the exact opposite of AV." It grants as many voters as possible at least one approved winner: it maximises score_CC(A,W) = |{i ∈ N : A(i) ∩ W ≠ ∅}| — the number of covered voters. (Introduced by Thiele, 1895; independently by Chamberlin & Courant, 1983.)
On this instance CC returns the unique {a, e, f, g} — the only size-4 committee that covers all 12 voters. The price of full coverage: CC seats e and g (one approval each) and drops b, c (four approvals each). Egalitarian coverage, purchased with low total satisfaction.
Rule 3 — Proportional Approval Voting (PAV): the middle¶
PAV is a Thiele method: each voter's satisfaction from a committee giving them j approved winners is the harmonic value 1 + 1/2 + ⋯ + 1/j, and PAV maximises the sum. The diminishing returns (1/j) mean a voter already well-represented counts for less, which produces proportional committees. Here PAV returns {a, b, c, f} — the popular a,b,c plus f for the f-faction, leaving 2 uncovered. seq-Phragmén, a load-balancing proportional rule, instead returns {a,b,c,d} on this profile — proportional rules need not agree.
The spectrum, on one election¶
| Rule | Character | Committee(s) | AV-score (Σ approvals) | Voters covered | Uncovered |
|---|---|---|---|---|---|
| AV | utilitarian | {a,b,c,d} | {a,b,c,f} |
18 | 9 | 10 | 3 | 2 |
| seq-Phragmén | proportional | {a,b,c,d} |
18 | 9 | 3 |
| PAV | proportional | {a,b,c,f} |
18 | 10 | 2 |
| SAV | beside the family | {a,b,c,f} |
18 | 10 | 2 |
| CC | egalitarian | {a,e,f,g} |
12 | 12 | 0 |
Reading down the table you trade total satisfaction (AV-score falls 18 → 12) for coverage (uncovered falls 3 → 0). AV sits at the utilitarian pole, CC at the egalitarian pole, and the proportional rules (PAV, Phragmén) live in between — exactly the spectrum the book uses this example to introduce.
The shadow-STAR bridge¶
Reading the same approval ballots as STAR ballots (approve = 5, else 0) and running STAR's committee rules (see the shadow-STAR page) maps the STAR family onto the same axis:
| STAR rule | lands like | committee |
|---|---|---|
| Bloc STAR | AV (utilitarian) | A,B,C,D |
| Allocated Score (STAR-PR) · SSS | AV-ish here (loosely proportional) | A,B,C,D |
| RRV | PAV (proportional) | A,B,C,F |
| (no direct analog) | CC (egalitarian coverage) | — |
Two lessons drop out: STAR's runoff adds nothing over AV on binary ballots (Bloc STAR = AV), and among STAR's proportional variants only RRV reaches the proportional PAV committee — while nothing in the STAR family targets CC's pure coverage objective.
Reproduce it¶
pip install abcvoting
python 06_Other/abcvoting_tabulation_engine/abc_tabulation.py \
04_Approval/02_Examples/multiwinner/cases/approval_bloc_4seats_c7_b12_lackner_skowron.yaml \
--rules av,sav,pav,cc,seqphragmen
# av -> {A,B,C,D}|{A,B,C,F} ; sav -> {A,B,C,F} ; pav -> {A,B,C,F}
# cc -> {A,E,F,G} ; seqphragmen -> {A,B,C,D}
Going deeper¶
AV, PAV, and CC are all one parameterised family — the Thiele methods — turned by a single satisfaction dial w. The formal treatment (the w-function spectrum, PAV's harmonic worked to 83/6, welfare vectors, seq-/rev-seq-PAV, and how RRV is the score-ballot cousin of seq-PAV) is in Thiele methods.
Not every approval rule is on the dial. Satisfaction Approval Voting (SAV) aims at the same coverage goal as CC for a fraction of the computation — but weights each voter by how many candidates they approved rather than how many of theirs won, which puts it beside the family rather than on it. On this instance it lands on {A,B,C,F}, resolving AV's D/F tie the same way PAV does: s(A)=4, s(B)=s(C)=11/6, s(F)=4/3, ahead of s(D)=s(E)=s(G)=1. It takes the paper's own examples to separate SAV from AV cleanly — there it elects a committee with no candidate in common.
References¶
- Lackner, M. & Skowron, P. (2023), Multi-Winner Voting with Approval Preferences, SpringerBriefs, doi:10.1007/978-3-031-09016-5 (open access) — §2.1–2.2, Examples 2.1–2.3, Rules 1–2.
- Thiele, T. N. (1895); Chamberlin, J. R. & Courant, P. N. (1983) — the CC rule.
abcvoting(Lackner et al.) — the peer-reviewed reference implementation used here.- Companion pages: 101 intro · shadow STAR · Approval — multi-winner.