1 Introduction
2 The multiple-district paradox
2.1 Case study
(a) Sub-group A | (b) Sub-group B | (c) Combined group (A + B) | |||||
---|---|---|---|---|---|---|---|
Expert | Ranking | Expert | Ranking | Expert | Ranking | Expert | Ranking |
\({e}_{{\mathrm{A}}_{1}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{B}}_{1}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{A}}_{1}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{B}}_{8}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) |
\({e}_{{\mathrm{A}}_{2}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{B}}_{2}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{A}}_{2}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{B}}_{9}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) |
\({e}_{{\mathrm{A}}_{3}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{B}}_{3}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{A}}_{3}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{B}}_{10}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) |
\({e}_{{\mathrm{A}}_{4}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{B}}_{4}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{A}}_{4}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{B}}_{11}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) |
\({e}_{{\mathrm{A}}_{5}}\) | \({O}_{2}\succ {O}_{1}\succ {O}_{3}\) | \({e}_{{\mathrm{B}}_{5}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{B}}_{1}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{B}}_{12}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) |
\({e}_{{\mathrm{A}}_{6}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) | \({e}_{{\mathrm{B}}_{6}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{B}}_{2}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{B}}_{13}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) |
\({e}_{{\mathrm{A}}_{7}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) | \({e}_{{\mathrm{B}}_{7}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) | \({e}_{{\mathrm{B}}_{3}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{B}}_{14}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) |
\({e}_{{\mathrm{A}}_{8}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) | \({e}_{{\mathrm{B}}_{8}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) | \({e}_{{\mathrm{B}}_{4}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{A}}_{11}}\) | \({O}_{3}\succ {O}_{1}\succ {O}_{2}\) |
\({e}_{{\mathrm{A}}_{9}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) | \({e}_{{\mathrm{B}}_{9}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) | \({e}_{{\mathrm{B}}_{5}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{A}}_{12}}\) | \({O}_{3}\succ {O}_{1}\succ {O}_{2}\) |
\({e}_{{\mathrm{A}}_{10}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) | \({e}_{{\mathrm{B}}_{10}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) | \({e}_{{\mathrm{B}}_{6}}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\) | \({e}_{{\mathrm{A}}_{13}}\) | \({O}_{3}\succ {O}_{1}\succ {O}_{2}\) |
\({e}_{{\mathrm{A}}_{11}}\) | \({O}_{3}\succ {O}_{1}\succ {O}_{2}\) | \({e}_{{\mathrm{B}}_{11}}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{1}\) | \({e}_{{\mathrm{A}}_{5}}\) | \({O}_{2}\succ {O}_{1}\succ {O}_{3}\) | \({e}_{{\mathrm{A}}_{14}}\) | \(O_{3} \succ O_{1} \succ O_{2}\) |
\(e_{{{\text{A}}_{12} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{B}}_{12} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{6} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{15} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) |
\(e_{{{\text{A}}_{13} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{B}}_{13} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{7} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{16} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) |
\(e_{{{\text{A}}_{14} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{B}}_{14} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{8} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{17} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) |
\(e_{{{\text{A}}_{15} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{B}}_{15} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{A}}_{9} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{B}}_{15} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) |
\(e_{{{\text{A}}_{16} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{A}}_{10} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | ||||
\(e_{{{\text{A}}_{17} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | \(e_{{{\text{B}}_{7} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) |
(d) Synthesis of the experts’ rankings | |||||
---|---|---|---|---|---|
Sub-group A | Sub-group B | Combined group (A + B) | |||
Expert | Ranking | Expert | Ranking | Expert | Ranking |
4 | \(O_{1} \succ O_{2} \succ O_{3}\) | 6 | \(O_{1} \succ O_{3} \succ O_{2}\) | 4 | \(O_{1} \succ O_{2} \succ O_{3}\) |
1 | \(O_{2} \succ O_{1} \succ O_{3}\) | 8 | \(O_{2} \succ O_{3} \succ O_{1}\) | 6 | \(O_{1} \succ O_{3} \succ O_{2}\) |
5 | \(O_{2} \succ O_{3} \succ O_{1}\) | 1 | \(O_{3} \succ O_{1} \succ O_{2}\) | 1 | \(O_{2} \succ O_{1} \succ O_{3}\) |
6 | \(O_{3} \succ O_{1} \succ O_{2}\) | 13 | \(O_{2} \succ O_{3} \succ O_{1}\) | ||
1 | \(O_{3} \succ O_{2} \succ O_{1}\) | 7 | \(O_{3} \succ O_{1} \succ O_{2}\) | ||
1 | \(O_{3} \succ O_{2} \succ O_{1}\) | ||||
Total: 17 | Total: 15 | Total: 32 |
-
In the first round (see Table 1-d), the design concept O1 obtains 4 first-choices, O2 obtains 6 first-choices, and O3 obtains 7 first-choices. Since no alternative has obtained more than half of the preferences based on first-choices, O1 – i.e., the alternative with fewest first-choices – is eliminated.
-
In the head-to-head comparison between O2 and O3, O2 obtains 10 first-choices while O3 obtains 7 first-choices. The winner is then O2.
-
The resulting collective ranking for sub-group A is: \(O_{2} \succ O_{3} \succ O_{1}\).Sub-group B
-
In the first round, the design concept O1 obtains 6 first-choices, O2 obtains 8 first-choices, and O3 obtains 1 first-choice. O2 obtains more than half of the preferences based on first-choices, while O3 is the one with fewest first-choices.
-
The resulting collective ranking for sub-group B is: \(O_{2} \succ O_{1} \succ O_{3}\).Combined groupAssuming that, ceteris paribus, the two expert sub-groups A and B are merged into a combined group (A + B) of thirty-two experts (see Table 1c), the IRV can be applied to all their merged rankings as follows.
-
In the first round, the design concept O1 obtains 10 first-choices, O2 obtains 14 first-choices, and O3 obtains 8 first-choices. Since no alternative has obtained more than half of the preferences based on first-choices, O3 – i.e., the one with fewest first-choices – is eliminated.
-
In the head-to-head comparison between O1 and O2, O1 obtains 17 first-choices while O2 obtains 15 first-choices. The winner is then O1.
-
The resulting collective ranking for the combined group (A + B) is: \(O_{1} \succ O_{2} \succ O_{3}\).The above results are paradoxical: considering both the two sub-groups A and B separately, the most preferred alternative is O2, while combining the two sub-groups, the most preferred alternative becomes O1. This result is difficult to justify since it is (at least apparently) contradictory and against logic: how could the team leader (or whoever) tolerate that – although O2 is the best design concept according to each individual sub-group – when combining the two sub-groups, O1 is the (new) best one?Table 2a summarizes the results obtained from the three previous applications of the IRV aggregation model.
No. of experts | (a) IRV | (b) Coombs’ | (c) BC | |
---|---|---|---|---|
Sub-group A | 17 |
\({\varvec{O}}_{2} \succ O_{3} \succ O_{1}\)
|
\(O_{3} \succ O_{1} \sim O_{2}\)
|
\(O_{3} \succ O_{2} \succ O_{1}\)
|
Sub-group B | 15 |
\({\varvec{O}}_{2} \succ O_{1} \succ O_{3}\)
|
\(O_{2} \succ O_{3} \succ O_{1}\)
|
\(O_{2} \sim O_{3} \succ O_{1}\)
|
Combined group (A + B) | 32 |
\({\varvec{O}}_{1} \succ O_{2} \succ O_{3}\)
|
\(O_{2} \succ O_{3} \succ O_{1}\)
|
\(O_{3} \succ O_{2} \succ O_{1}\)
|
2.2 Changing aggregation model and preference profile
2.2.1 Coombs’ aggregation model
2.2.2 Borda count model
2.2.3 Further case study
(a) Sub-group A’ | (b) Sub-group B’ | (c) Combined group (A’ + B’) | |||||||
---|---|---|---|---|---|---|---|---|---|
Expert | Ranking | Expert | Ranking | Expert | Ranking | Expert | Ranking | Expert | Ranking |
\({e}_{{\mathrm{A}}_{1}^{^{\prime}}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{A}}_{22}^{^{\prime}}}\) | \({O}_{3}\succ {O}_{1}\succ {O}_{2}\) | \({e}_{{\mathrm{B}}_{1}^{^{\prime}}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{A}}_{1}^{^{\prime}}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{A}}_{21}^{^{\prime}}}\) | \({O}_{3}\succ {O}_{1}\succ {O}_{2}\) |
\({e}_{{\mathrm{A}}_{2}^{^{\prime}}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{A}}_{23}^{^{\prime}}}\) | \({O}_{3}\succ {O}_{1}\succ {O}_{2}\) | \({e}_{{\mathrm{B}}_{2}^{^{\prime}}}\) | \({O}_{2}\succ {O}_{1}\succ {O}_{3}\) | \({e}_{{\mathrm{A}}_{2}^{^{\prime}}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{A}}_{22}^{^{\prime}}}\) | \({O}_{3}\succ {O}_{1}\succ {O}_{2}\) |
\({e}_{{\mathrm{A}}_{3}^{^{\prime}}}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\) | \({e}_{{\mathrm{A}}_{24}^{^{\prime}}}\) | \({O}_{3}\succ {O}_{1}\succ {O}_{2}\) | \({e}_{{\mathrm{B}}_{3}^{^{\prime}}}\) | \({O}_{2}\succ {O}_{1}\succ {O}_{3}\) | \({e}_{{\mathrm{A}}_{3}^{^{\prime}}}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{23}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) |
\(e_{{{\text{A}}_{4}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{25}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{B}}_{4}^{^{\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3}\) | \(e_{{{\text{A}}_{4}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{24}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) |
\(e_{{{\text{A}}_{5}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{26}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{B}}_{5}^{^{\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3}\) | \(e_{{{\text{A}}_{5}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{25}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) |
\(e_{{{\text{A}}_{6}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{27}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{B}}_{6}^{^{\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3}\) | \(e_{{{\text{A}}_{6}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{26}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) |
\(e_{{{\text{A}}_{7}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{28}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{B}}_{7}^{^{\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3}\) | \(e_{{{\text{A}}_{7}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{27}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) |
\(e_{{{\text{A}}_{8}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{29}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{A}}_{8}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{28}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | ||
\(e_{{{\text{A}}_{9}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{30}^{^{\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | \(e_{{{\text{A}}_{9}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{29}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | ||
\(e_{{{\text{A}}_{10}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{31}^{^{\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | \(e_{{{\text{B}}_{1}^{^{\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{30}^{^{\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | ||
\(e_{{{\text{A}}_{11}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{32}^{^{\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | \(e_{{{\text{A}}_{10}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{31}^{^{\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | ||
\(e_{{{\text{A}}_{12}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{33}^{^{\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | \(e_{{{\text{A}}_{11}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{32}^{^{\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | ||
\(e_{{{\text{A}}_{13}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{34}^{^{\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | \(e_{{{\text{A}}_{12}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{33}^{^{\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | ||
\(e_{{{\text{A}}_{14}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{13}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{34}^{^{\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1}\) | ||||
\(e_{{{\text{A}}_{15}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{14}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{B}}_{2}^{^{\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3}\) | ||||
\(e_{{{\text{A}}_{16}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{B}}_{3}^{^{\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3}\) | |||||
\(e_{{{\text{A}}_{17}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{16}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{B}}_{4}^{^{\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3}\) | ||||
\(e_{{{\text{A}}_{18}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{A}}_{17}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{B}}_{5}^{^{\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3}\) | ||||
\(e_{{{\text{A}}_{19}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{A}}_{18}^{^{\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1}\) | \(e_{{{\text{B}}_{6}^{^{\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3}\) | ||||
\(e_{{{\text{A}}_{20}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{A}}_{19}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{B}}_{7}^{^{\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3}\) | ||||
\(e_{{{\text{A}}_{21}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) | \(e_{{{\text{A}}_{20}^{^{\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2}\) |
(d) Synthesis of the experts’ rankings | |||||
---|---|---|---|---|---|
Sub-group A | Sub-group B | Combined group (A + B) | |||
Expert | Ranking | Expert | Ranking | Expert | Ranking |
9 | \(O_{1} \succ O_{2} \succ O_{3}\) | 1 | \(O_{1} \succ O_{2} \succ O_{3}\) | 10 | \(O_{1} \succ O_{2} \succ O_{3}\) |
9 | \(O_{2} \succ O_{3} \succ O_{1}\) | 6 | \(O_{2} \succ O_{1} \succ O_{3}\) | 6 | \(O_{2} \succ O_{1} \succ O_{3}\) |
11 | \(O_{3} \succ O_{1} \succ O_{2}\) | 9 | \(O_{2} \succ O_{3} \succ O_{1}\) | ||
5 | \(O_{3} \succ O_{2} \succ O_{1}\) | 11 | \(O_{3} \succ O_{1} \succ O_{2}\) | ||
5 | \(O_{3} \succ O_{2} \succ O_{1}\) | ||||
Total: 34 | Total: 7 | Total: 41 |
No. of experts | (a) IRV | (b) Coombs’ | (c) BC | |
---|---|---|---|---|
Sub-group A’ | 34 | \(O_{3} \succ O_{1} \sim O_{2}\) | \({\varvec{O}}_{2} \succ O_{3} \succ O_{1}\) | \(O_{3} \succ O_{2} \succ O_{1}\) |
Sub-group B’ | 7 | \(O_{2} \succ O_{1} \succ O_{3}\) | \({\varvec{O}}_{2} \succ O_{1} \succ O_{3}\) | \(O_{2} \succ O_{1} \succ O_{3}\) |
Combined group (A’ and B’) | 41 | \(O_{2} \succ O_{3} \succ O_{1}\) | \({\varvec{O}}_{1} \succ O_{2} \succ O_{3}\) | \(O_{2} \succ O_{3} \succ O_{1}\) |
2.3 Triggering factors of the paradox
3 Methodology
-
The first one briefly recalls the aforementioned indicators;
-
The second one illustrates the use of these indicators for decision-making problems involving rankings with a relatively limited number of alternatives (as those previously exemplified);
-
The third part shows a step-by-step technique – denominated technique of partialized rankings – able to identify the potential triggering reasons of the paradox.
3.1 Concordance and coherence indicators
3.2 Interpretation of the paradox
k-th model | (Sub-)group | No. of experts | Collect. ranking | \(W^{\left( m \right)}\) | \(W_{k}^{{\left( {m + 1} \right)}}\) | \(b_{k}^{\left( m \right)}\) |
---|---|---|---|---|---|---|
(a) | ||||||
IRV | Sub-group A | 17 | \({\varvec{O}}_{2} \succ O_{3} \succ O_{1}\) | 1.38% | 2.16% | 1.56 |
Sub-group B | 15 | \({\varvec{O}}_{2} \succ O_{1} \succ O_{3}\) | 1.35% | 1.56% | 1.15 | |
Combined group (A and B) | 32 | \({\varvec{O}}_{1} \succ O_{2} \succ O_{3}\) | 1.27% | 0.64% | 0.51 | |
(b) | ||||||
Coombs’ | Sub-group A | 17 | \(O_{3} \succ O_{1} \sim O_{2}\) | 1.38% | 2.42% | 1.75 |
Sub-group B | 15 | \(O_{2} \succ O_{3} \succ O_{1}\) | 1.35% | 2.73% | 2.02 | |
Combined group (A and B) | 32 | \(O_{2} \succ O_{3} \succ O_{1}\) | 1.27% | 1.70% | 1.34 | |
(c) | ||||||
BC | Sub-group A | 17 | \(O_{3} \succ O_{2} \succ O_{1}\) | 1.38% | 2.77% | 2.00 |
Sub-group B | 15 | \(O_{2} \sim O_{3} \succ O_{1}\) | 1.35% | 2.68% | 1.98 | |
Combined group (A and B) | 32 | \(O_{3} \succ O_{2} \succ O_{1}\) | 1.27% | 1.93% | 1.52 |
k-th model | (Sub-)group | No. of experts | Collect. ranking | \(W^{\left( m \right)}\) | \(W_{k}^{{\left( {m + 1} \right)}}\) | \(b_{k}^{\left( m \right)}\) |
---|---|---|---|---|---|---|
(a) | ||||||
IRV | Sub-group A’ | 34 | \(O_{3} \succ O_{1} { }\sim { }O_{2}\) | 3.37% | 4.13% | 1.22 |
Sub-group B’ | 7 | \(O_{2} \succ O_{1} \succ O_{3}\) | 87.7% | 89.0% | 1.01 | |
Combined group (A’ and B’) | 41 | \(O_{2} \succ O_{3} \succ O_{1}\) | 0.95% | 1.42% | 1.49 | |
(b) | ||||||
Coombs’ | Sub-group A’ | 34 | \({\varvec{O}}_{2} \succ O_{3} \succ O_{1}\) | 3.37% | 3.51% | 1.04 |
Sub-group B’ | 7 | \({\varvec{O}}_{2} \succ O_{1} \succ O_{3}\) | 87.7% | 89.0% | 1.01 | |
Combined group (A’ and B’) | 41 | \({\varvec{O}}_{1} \succ O_{2} \succ O_{3}\) | 0.95% | 0.74% | 0.77 | |
(c) | ||||||
BC | Sub-group A’ | 34 | \(O_{3} \succ O_{2} \succ O_{1}\) | 3.37% | 4.24% | 1.26 |
Sub-group B’ | 7 | \(O_{2} \succ O_{1} \succ O_{3}\) | 87.7% | 89.0% | 1.01 | |
Combined group (A’ and B’) | 41 | \(O_{2} \succ O_{3} \succ O_{1}\) | 0.95% | 1.42% | 1.49 |
3.3 The technique of partialized rankings
(a) Sub-group A’’ | (b) Sub-group B’’ | (c) Combined group (A’’ + B’’) | |||||
---|---|---|---|---|---|---|---|
Expert | Ranking | Expert | Ranking | Expert | Ranking | Expert | Ranking |
\(e_{{{\text{A}}_{1}^{^{\prime\prime}} }}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\succ {O}_{4}\) | \(e_{{B_{1}^{^{\prime\prime}} }}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\succ {O}_{4}\) | \(e_{{{\text{A}}_{1}^{^{\prime\prime}} }}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\succ {O}_{4}\) | \(e_{{{\text{A}}_{7}^{^{\prime\prime}} }}\) | \({O}_{2}\succ {O}_{3}\succ {O}_{4}\succ {O}_{1}\) |
\(e_{{{\text{A}}_{2}^{^{\prime\prime}} }}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\succ {O}_{4}\) | \(e_{{B_{2}^{^{\prime\prime}} }}\) | \({O}_{1}\succ {O}_{3}\succ {O}_{2}\succ {O}_{4}\) | \(e_{{{\text{A}}_{2}^{^{\prime\prime}} }}\) | \({O}_{1}\succ {O}_{2}\succ {O}_{3}\succ {O}_{4}\) | \(e_{{{\text{A}}_{12}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{3}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3} \succ O_{4}\) | \(e_{{{\text{B}}_{3}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{3} \succ O_{2} \succ O_{4}\) | \(e_{{{\text{A}}_{3}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{2} \succ O_{3} \succ O_{4}\) | \(e_{{{\text{A}}_{14}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{4}^{^{\prime\prime}} }}\) | \(O_{4} \succ O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{B}}_{4}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{3} \succ O_{2} \succ O_{4}\) | \(e_{{{\text{A}}_{4}^{^{\prime\prime}} }}\) | \(O_{4} \succ O_{1} \succ O_{2} \succ O_{3}\) | \(e_{{{\text{A}}_{15}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{5}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3} \succ O_{4}\) | \(e_{{{\text{B}}_{5}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{3} \succ O_{2} \succ O_{4}\) | \(e_{{{\text{A}}_{5}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{1} \succ O_{3} \succ O_{4}\) | \(e_{{{\text{A}}_{16}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{6}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{6}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{3} \succ O_{2} \succ O_{4}\) | \(e_{{{\text{A}}_{6}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{A}}_{17}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1} \succ O_{4}\) |
\(e_{{{\text{A}}_{7}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{4} \succ O_{1}\) | \(e_{{{\text{B}}_{7}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{A}}_{8}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{A}}_{11}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{8}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{8}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{A}}_{9}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{15}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{9}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{9}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{A}}_{10}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{A}}_{13}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{4} \succ O_{2}\) |
\(e_{{{\text{A}}_{10}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{10}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{7}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{1}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{3} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{11}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) | \(e_{{{\text{B}}_{11}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{8}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{2}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{3} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{12}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) | \(e_{{{\text{B}}_{12}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{9}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{3}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{3} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{13}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{4} \succ O_{2}\) | \(e_{{{\text{B}}_{13}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{10}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{4}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{3} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{14}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) | \(e_{{{\text{B}}_{14}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{11}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{5}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{3} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{15}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) | \(e_{{{\text{B}}_{15}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) | \(e_{{{\text{B}}_{12}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{6}^{^{\prime\prime}} }}\) | \(O_{1} \succ O_{3} \succ O_{2} \succ O_{4}\) |
\(e_{{{\text{A}}_{16}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{1} \succ O_{2} \succ O_{4}\) | \(e_{{{\text{B}}_{13}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | ||||
\(e_{{{\text{A}}_{17}^{^{\prime\prime}} }}\) | \(O_{3} \succ O_{2} \succ O_{1} \succ O_{4}\) | \(e_{{{\text{B}}_{14}^{^{\prime\prime}} }}\) | \(O_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) |
Step | (Sub-)group | No. of experts | (Partialized) collect. ranking | \(W^{\left( m \right)}\) | \(W_{k}^{{\left( {m + 1} \right)}}\) | \(b_{k}^{\left( m \right)}\) |
---|---|---|---|---|---|---|
(Step 1) “Partialized” rankings excluding O3 and O4 | Sub-group A’’ | 17 | \(O_{1} \succ O_{2}\) | 3.11% | 4.94% | 1.586 |
Sub-group B’’ | 15 | \(O_{2} \succ O_{1}\) | 0.44% | 1.56% | 3.516 | |
Combined group | 32 | \(O_{1} \succ O_{2}\) | 0.39% | 0.83% | 2.116 | |
(Step 2) “Partialized” rankings excluding O4 | Sub-group A’’ | 17 | \({\varvec{O}}_{2} \succ O_{3} \succ O_{1}\) | 1.38% | 2.16% | 1.561 |
Sub-group B’’ | 15 | \({\varvec{O}}_{2} \succ O_{1} \succ O_{3}\) | 1.33% | 1.56% | 1.172 | |
Combined group | 32 | \({\varvec{O}}_{1} \succ O_{2} \succ O_{3}\) | 1.27% | 0.64% | 0.506 | |
(Step 3) Complete rankings | Sub-group A’’ | 17 | \({\varvec{O}}_{2} \succ O_{3} \succ O_{1} \succ O_{4}\) | 39.65% | 40.99% | 1.034 |
Sub-group B’’ | 15 | \({\varvec{O}}_{2} \succ O_{1} \succ O_{3} \succ O_{4}\) | 60.53% | 60.63% | 1.002 | |
Combined group | 32 | \({\varvec{O}}_{1} \succ O_{2} \succ O_{3} \succ O_{4}\) | 48.79% | 48.83% | 1.001 |
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The degree of concordance between expert rankings is not as dramatically low as in the previous examples. The introduction of the new alternative O4, which is typically placed by the experts in the bottom positions, contributes to increase the \(W^{\left( m \right)}\) indicator compared to the example in Table 1, "masking" the discordance related to the positioning of O1, O2 and O3. Indicators are sensitive to the presence of all alternatives and to the so-called “irrelevant alternatives” too [22, 23].
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Despite the occurrence of the paradox, \(W_{k}^{{\left( {m + 1} \right)}} \ge W^{\left( m \right)}\) and \(b_{k}^{\left( m \right)} \ge 1\), denoting positive coherence between (the three) collective rankings and the relevant expert rankings; again, the (local) incoherence due to the presence of the paradox seems to be compensated by a relative coherence of the alternatives in non-top positions. In this case, \(W^{\left( m \right)}\), \(W_{k}^{{\left( {m + 1} \right)}}\) and \(b_{k}^{\left( m \right)}\) do not “respond” to the paradox, which only concerns the top alternatives (O1 and O2).
4 Conclusions
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The occurrence of the paradox is typically associated with a very low degree of concordance among the expert rankings, with particular reference to the alternatives in the top positions.
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The occurrence of the paradox may concern different aggregation models, depending on the specific (i) preference profile and (ii) repartition of the rankings into sub-groups.
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The choice of a method to aggregate the expert rankings into a collective one may affects the results even more than the preference profile.
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Some aggregation models, classified as PSPs, are “immune” from the multiple-district paradox [43].
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\(W^{\left( m \right)}\), which measures the concordance between expert rankings;
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\(W_{k}^{{\left( {m + 1} \right)}}\) and \(b_{k}^{\left( m \right)}\), which measure the consistency between the expert rankings and the collective ranking obtained through a certain (k-th) aggregation model.
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The proposed methodology is based on application of specific (concordance and consistency) indicators. The choice of other indicators could lead to (at least partially) different outcomes [54].
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Although the multiple-district paradox is especially interesting for design decision-making problems in which the best alternative should be determined, it remains one-and-one-only of the possible paradoxes documented in the scientific literature; e.g., other paradoxes are the so-called no-shows, preference inversion, absolute majority loser, etc. [34].