I looked at all the by-elections going back to 1998 to see if I could see any trends in turnout in by-elections and what this might mean for the Victoria, Calgary Centre and Durham by-elections. I am not going to do this with Etobicoke Centre because if there is a by-election there it will be a very different race.
Average by-election turnout in 40 by-elctions - 33.4%
Average turnout in the general election before the by-election - 61.2%
Turnout at the by-election averages 54.6% of the previous general election.
When we look at the 13 races where the riding changed hands and several other competitive races the average by-election turnout was 36.4% and the pervious general election before 59.4%. In the close races the turnout averages 61.3% of the previous general election
The same calculation for the non-noncompetitive ridings gives us 50.3% of the general election turnout in the by-election.
What the total votes could look like
How competitive are the ridings?
Calgary Centre has never been won by anyone not on the right. Joe Clark is on the right before anyone points to him. Looking at the race it strikes me the real race is who will come second.
Durham federally has been held by the Liberals federally but never provincially. The Liberals benefited from the vote split between the PCs and Canadian Alliance/Reform. It does not strike me as a realistically competitive riding.
Victoria was held by the Liberals from 1993 to 2006 - in only one of those elections did they get more than 40% of the vote. In every election in the last two generations other than 2004 and 2006 the right has either won or come second. The NDP have four of the last eight elections in Victoria. Denise Savoie did win last time with 50.8% of the vote, but only managed 38.5% in her first win.
Provincially most of the riding is represented by the NDP, but the eastern part has been held by the BC Liberals since 1996.
I think Victoria is marginally competitive.
There seems to some co-relation between higher turnout in by-elections and location, but there are not really enough data points to be certain. Urban ridings and ones in Ontario have these mild co-relations but it is hard to know if there is a cause and effect relationship there.
Calgary Centre 25,000 27.8%
Durham 29,000 31.5%
Victoria 36,000 40.4%
UPDATED November 15th
Calgary Centre 35,000 38.9%
Durham 29,000 31.5%
Victoria 37,000 41.5%