Factors that come into play:
- If it is a close race or changes hands the drop in voter turn out is less but there is still a drop
- There is a weak co-relation between better turnout in Ontario and BC and worse ones in Quebec. For the rest of the country the data is not clear enough
Taking the past results into account from 56 federal by-elections here is my estimate of the turnout for each of the pending by-elections
- Bourassa 29% (28-30%) 20,500 voters
- Toronto Centre 29% (26%-32%) 25,500 - the overall data indicates higher but the actual results from 2008 indicate lower
- Provencher 36% (34-38%) 22,400 voters
The only wildcard I can see in all of this is a serious campaign by the Greens in Bourassa. When the Greens had full campaigns in London North Centre in 2006 and Victoria in 2012 the voter turnout was higher than the past data would indicate. In both cases it was roughly higher in direct proportion to the Green vote increase over the previous general election. With only two data points to work with it is really hard to know how to account for this possible effect. It might add an extra 5000 to 6000 votes to the Bourassa turnout taking the turnout to around 37%.