A record of forecasts vs. results can be found below - the barometer has only made the wrong call five times in 51 federal and provincial by-elections since 2012, for an accuracy rating of 90.2%.
Three of the four errors have occurred in Newfoundland and Labrador, where the Liberals have gone from a tiny share of the vote in 2011 to big by-election victories. In one of the ridings, there was a swing of almost 90 points between the Tories and the Liberals.
In one of the other errors, the Progressive Conservatives won the Alberta riding of Calgary West by 2.8 points over Wildrose, who were favoured. In the model, Wildrose was given a 65% chance of winning, with the Tories at 35%.
In the other, the Liberals defeated the Coalition Avenir Québec in the riding of Chauveau. A popular former MNA, Gérard Deltell, had resigned, and the Liberals had a star candidate to go up against the star candidate that the CAQ found.
What the By-Election Barometer is
The By-Election Barometer tracks all scheduled and upcoming federal and provincial by-elections. The percentages shown in the charts represent margins, colour-coded according to the standards adopted by this site. The first set show the results of the last two elections.
The second set of margins are those that the projection model churns out when each regional/provincial poll is applied, using the same system as ThreeHundredEight's standard seat projection model. These are a way to demonstrate what might be expected in the riding, based on regional trends. The rolling 30-day average calculates an unweighted average of these projections with the last 30-days of regional polling.
The third set of margins, when available, represent the results of actual polls of the riding that have been released.
Finally, ThreeHundredEight's Forecast is calculated by taking the projected range of results from the average of the last 30 days of polling, including any polls done for the riding itself, and comparing how the ranges for each party overlap. The amount of overlap that potentially puts a party in a position to win is then tallied, the result being a percentage "chance" of that party winning the riding.
A Strong result means a 95% to 100% chance of winning, Likely is a 75% to 94% chance, Lean is a 60% to 74% chance, and Toss-Up means the chances of a party winning are 59% or less.
The Wildcard section describes any factor that could make the result unpredictable.
What it isn't
The By-Election Barometer is not a poll, the section titled "Margin after application of swing from regional polls" is not a list of riding polls, and the 30-day average is not a projection. By-Elections are notoriously hard to call, and the Barometer is not a tracking of actual voting intentions. The forecast is also not an opinion. As always, I am tied to what the numbers show.
By-elections have been called for July 14 for the provincial ridings of Dartmouth South, Sydney-Whitney Pier, and Cape Breton Centre in Nova Scotia.
Federal by-elections have been called for the ridings of Ottawa-West Nepean, Peterborough, and Sudbury for October 19. As they will be superseded by the federal election, I will not cover them here.
The riding has a rather kaleidoscope history, having been held by the Liberals, New Democrats, and Tories at some point during the 1990s.
As Dartmouth South-Portland Valley, however, the riding swung towards the New Democrats in dramatic fashion. The party increased its share from 43% in 2003 to 48% in 2006 and finally 55% in 2009. The Liberals placed third in 2009 with 29%.
But in the last election, the riding swung heavily towards the Liberals, as occurred province wide. From a 28-point NDP victory (on the transposed boundaries), the Liberals won it by 13 points with 46% to 33% for the NDP. The Tories were third with 18%.
With the Liberals still leading very comfortably in the polls, Dartmouth South will probably remain in Liberal hands.
Forecast history: STRONG LIBERAL since Rowe's passing, became LIKELY LIBERAL on June 12.
The riding had been a very safe one for the NDP, as they won it in 2009 with 80% of the vote. He more normally took between 40% and 60% of the vote, but in 2013, as the NDP was swept from power, his margin of victory fell to just two points: 45% to 43% for the Liberals.
In most elections, the Liberals were Corbett's chief rival and with their poll numbers up they stand a good chance to take the riding they last held between 1988 and 1998.
But ridings being relatively small in Nova Scotia, and Cape Breton being a place apart, a good candidate could potentially win it for any party. The Tories, for instance, finished second in 2006.
Forecast history: STRONG LIBERAL since Corbett's resignation, became TOSS-UP (NDP/LIB) on June 12.
Gosse just held on to the riding in 2013 with 49% of the vote against 44% for the Liberal candidate, after having won landslides in the predecessor riding of Cape Breton Nova in 2006 and 2009.
But the riding could now be won by the Liberals, who are up in the polls in Nova Scotia. And while Cape Breton Nova was Liberal from 1990 to 2003, the other predecessor riding, Cape Breton South, had voted Liberal in every election starting in 1974. The roots are there for a Liberal return to the riding - as well as a continuation of the NDP.
Forecast history: STRONG LIBERAL since Gosse's resignation, became TOSS-UP (NDP/LIB) on June 12.