Is PR-STV proportional? (Yes).
We hear quite frequently in the media and even from organizations with a history of campaigning for electoral reform that "STV is not proportional."
For example, CUPE, who is part of the Every Vote Counts Alliance for PR, recently passed a resolution that stated they plan to:
"Oppose ranked ballots, preferential ballots and other single transferable vote systems and work to ensure that members and the general public understand that they are not true proportional representation;"
This is a troubling message to be reading when PR supporters must united to convince the all-party Parliamentary committee to choose a proportional system.
A google search for "Single Transferable Vote" brings up Wikipedia as our first hit, previewing the following content:
The single transferable vote (STV) is a voting system designed to achieve proportional representation through ranked voting in multi-seat constituencies (voting districts).
PR-STV is not AV (Alternative Vote)
A big part of the confusion about PR-STV among the general public is the term "ranked ballot." The media love to talk about "the ranked ballot system."
A "ranked ballot" is not a voting system.
It's just a tool, or feature.
When the media uses the term "ranked ballot" they use it as a shortcut to refer to a particular voting system used in Australia which is properly called Alternative Vote (AV) which uses a ranked ballot.
Now check out the chart in this picture from the Institute for Democracy and Electoral Assistance (IDEA). Take a moment to find STV and AV on the electoral systems families chart.
Note that STV is in the "proportional representation" family.
AV is in the "plurality/majority" family.
Systems in these two families are designed based on very different principles. AV is a close cousin of first-past-the-post, and produces equally disproportional results. PR-STV should not get swept into the winner-take-all family by anyone, period.
How Proportional Is PR-STV?
Like any PR model, it depends entirely on the design.
None of the proportional models on the table for Canada are going to give us proportional results like you might see in Denmark, Sweden and New Zealand.
Index of Disproportionality
The Index of Disproportionality is what political scientists use to measure how disproportional election results are.The higher the number on the Index, the more disproportional the results. To put this on a scale, Denmark and the Netherlands score a 1 and Canada scores an average of 10.79.
Below is the average score in the last 4 elections from countries with MMP or PR-STV with designs roughly equivalent to what we might see in Canada.
Ireland (STV - 3-5 seat districts): 6.69
Tasmania (STV - 5 seat districts): 4.89
Scotland (MMP - regional lists): 6.83
But with our current parties, check out this comparison of MMP (14 member regions and 8 member regions) and PR-STV and STV+ based on the votes cast in October - not much difference.
A Special Note About STV and What "Proportionality" Means
Some scholars have had trouble properly classifying STV.
Remember, STV was invented to reflect diversity ("proportionality") before there were political parties.
But the Index of Disproportionality measures proportionality by parties.
When people explain proportional representation, the easiest shortcut to describe it is in terms of votes for parties: "If a party gets 30% of the vote, they get 30% of the seats."
Proportional systems have been created such as List PR and MMP which are designed to achieve and measure proportional results only along party lines.
STV wasn't designed around parties - it was designed around candidates.
It proportionally reflects whatever kind of diversity exists in a district. It is designed to reflect whatever is most important to the voters.
So if everybody voted primarily based not on party preferences but on what hair colours the candidates had, STV would accurately reflect how many people preferred different hair colours.
In today's world, most people do vote along party lines - their first choice candidate is likely from their first choice party. So STV produces proportional results by party affiliation as a natural result.
But STV leaves room for voters to decide what other things are most important to them in making choices: including the characteristics of individual candidates. It even gives a popular independent a chance of being elected.