Methodology

The introduction leaves many questions unanswered, primarily about methodology. We shall address them as follows:

  1. What are the basic assumptions behind this methodology?
  2. How is the REScore for each voter calculated?
  3. Are there alternative calculations that should be considered?
  4. How are Representation Equity Scores for individual voters used to generate the Representation Equity Index?
  5. How are voting preferences simulated, given the lack of such data for Canadian elections?

Assumptions

The core assumptions are that each voter should have a representative1 they can turn to with their policy concerns and that equal representation is better than unequal representation.

We assume that voting uses a ranked ballot (not necessarily exhaustive) where the maximum number of preferences can be specified. Single Member Plurality simply sets the maximum number of preferences to one. Using a common ballot format allows us to more easily compare several different electoral systems, although modelling ranked ballots in the Canadian context is difficult.

We assume that each voter ranks their ballot sincerely and that a higher-ranked candidate will represent the voter better than a lower-ranked candidate.

Calculating the Representation Effectiveness Score for each voter

This calculation involves two steps. The first step is to apply a utility function, u(b,e,w) to each voter. The utility function consumes

  • a ranked ballot, b,
  • the set of elected candidates, e, that could have appeared on the voter’s ballot (i.e. that the voter could have voted for),
  • and a vector of weights, w.

It produces a non-negative number where 0.0 indicates no substantive representation for that voter and larger numbers indicate better representation. This function is discussed in more depth in a moment.

The second step is to normalize these values so that 1.0 is the average representation. 0.0 still means no representation and larger values still mean better representation. The REScore equation for the ith voter is

REScore

where:

  • n is the number of voters
  • bi is the ballot for the ith voter.

That is, it’s simply the voter’s utility function divided by the average of the function applied to all voters.

There are many possible implementations of the utility function, u(b,e,w). The ones we have considered sum the contributions from each elected candidate on the voter's ballot, weighted according to how they were ranked.

Mathematically, the utility function is implemented as follows

Utility function

where

  • b is the ranked ballot cast by the voter and bi is the candidate ranked as the _i_th preference. Note that this use of bi is different than the one above. [Suggestions?]
  • e is the set of elected candidates that could have appeared on b.
  • w is a vector of weights in [0, 1] giving the contribution of each preference.
  • m is the number of preferences expressed on ballot b.
  • g(|bi|) is a function on the number of votes received by candidate bi.

In plain English, we sum the contribution of each successful candidate on the voter’s ballot. Each of those contributions is weighted, presumably with the first preference being weighted more heavily than the second preference, which is weighted more heavily than the third. Finally, the contribution is divided by the number of voters that candidate represents. Representing fewer voters means the candidate can give each one more attention and thus represent them better.2 Most of our work to date has divided by the square root of the number of votes.

Calculating the Representation Equity Index for an election

The Representation Equity Index is a single number that captures the equity (or inequity) of the Representation Effectiveness Scores. It's caculated exactly the same way as the Gini Coeffiencent that is commonly used to represent income inequality.

Simulating voter preferences

Simulating voter preferences is the same as generating a ranked ballot for each voter. This problem is discussed in the next section.


  1. We distinguish between the representation work of an MP from the constituency work of an MP. The former consists of representing a voter’s policy concerns in Parliament. The latter consists of helping the voter navigate government bureaucracy. We are interested in the former and uninterested in the latter as it could be easily replaced by a non-partisan ombudsman. ↩︎

  2. This is taking into account, roughly, the issues that gave rise to the “one person, one vote” Supreme Court case in the United States. ↩︎