Should I rank all the candidates in BC-STV?

April 14, 2009

Under the proposed new electoral system for British Columbia, BC-STV, you rank candidates rather than choose a single winner.  You may rank as many or as few candidates as you wish but people often ask “Should I rank everyone or is it better to just rank a few or one?”

If you are interested in every possible amount of your vote counting then the short answer is, you should rank everyone.  This is because the only way for your vote to be wasted under BC-STV is if your vote has an opportunity to be transferred and you have no further preferences listed.  Lets say you only rank one candidate, the person you really want to win, candidate Alice.  Now if Alice wins with twice as many votes as she needed, then everyone who voted for her will get half their vote transferred to their second (or third, or fouth…) choice.  Since you didn’t list any second choice, half your vote is now thrown away.  You did help elect Alice in exactly the same way you do under the current voting system.  But just like the current voting system, after that your vote doesn’t count anymore.

Maybe this doesn’t seem so bad to you, but think about the opposite case.  Lets say Alice was actually very unpopular and in order for other candidates to get enough votes to be elected, Alice was eliminated during counting.  Now everyone who voted for Alice as their first choice gets to transferred to their second choice at full strength.  Since you didn’t put down a second choice, your ballot is thrown away and 100% of your vote is now wasted, just like under first past the post.  So if you like the current system, and want to keep voting that way by all means, simply select your first choice and no others.

Now, the situation isn’t as bad if you list 4 or 5 preferences or just leave a few candidates unselected.  Then the amount you are risking to waste will be quite small.  But if you work through it logically you can show that the only time when it doesn’t matter if rank the remaining candidates is when you are completely indifferent between all of them.  That means, if there are 3 people left on your ballot that you have not ranked yet and you think they are equally bad choices then leaving them unnumbered doesn’t hurt you.  But if you think one of them, candidate Fluff, is slightly less bad than the other two, Goof and Hump, its still rational to rank Fluff and leave the other two blank even if you think Fluff is a horrible, horrible candidate.  There is no risk that you are helping Fluff win because your choice for Frank over Goof and Hump is only used if the following things have already happened to each of your previous ranked candidates

  • they’ve been elected to a seat
  • they’ve been eliminated because they are less popular than Fluff, Goof and Hump

So at that stage, why not have a say over which of these horrible candidates gets in?  As I said, if you really can’t distinguish them in their horribleness then randomly ordering them will not be more meaningful than wasting your vote so you can just not rank them at all.

I hope that clears it up.

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