Social utility efficiency

Social utility efficiency (SUE) is a measurement of the utilitarian performance of voting methods—how likely they are to elect the candidate who best represents the voters' preferences.

It is also known as utilitarian efficiency, voter satisfaction index (VSI) or voter satisfaction efficiency (VSE).

Definition
Social utility efficiency is defined as the ratio between the social utility of the candidate who is actually elected by a given voting method and that of the candidate who would maximize social utility, where $$E[]$$is the expected value over many iterations of the sum of all voter utilities for a given candidate:

$$\operatorname{SUE}= \frac{E[\text{selected candidate}]-E[\text{random candidate}]}{E[\text{maximizing candidate}]-E[\text{random candidate}]}$$

A voting method with 100% efficiency would always pick the candidate that maximizes voter utility. A method that chooses a winner randomly would have efficiency of 0%, and a (pathological) method that did worse than a random pick would have less than 0% efficiency.

SUE is not only affected by the voting method, but is a function of the number of voters, number of candidates, and of any strategies used by the voters.

History
The concept was originally introduced as a system's "effectiveness" by Robert J. Weber in 1977, defined as:

$$\operatorname{Effectiveness}=\lim_{n \to \infty} \frac {E_\text{elected}(m,n) - E_\text{random}(m)}{E_\text{maximal}(m,n) - E_\text{random}(m)}$$

Where $$E$$ is the expected social utility of the given candidate, $$n$$ is the number of voters, and $$m$$ is the number of candidates. He used a random society (impartial culture) model to analytically calculate the effectiveness of FPTP, two Approval variants, and Borda, as the number of voters approaches infinity.

It was given the name "social utility efficiency" and extended to the more realistic spatial model of voting by Samuel Merrill III in the 1980s, calculated statistically from random samples, with 25–201 voters and 2–10 candidates. This analysis included FPTP, Runoff, IRV, Coombs, Approval, Black, and Borda (in increasing order of efficiency). (Merrill's model normalizes individual voter utility before finding the utility winner, while Weber's does not, so that Merrill considers all 2-candidate voting systems to have an SUE of 100%, decreasing with more candidates, while Weber considers them to have an effectiveness of $$\sqrt{2/3}$$ = 81.6%, with some systems increasing with more candidates.)