How the Poll Tracker works

The full methodology for Éric Grenier's vote and seat projections.

The full methodology for Éric Grenier's vote and seat projections

How can you project an election? (Dave Gilson/CBC)

The following is a detailed explanation of the vote and seat projection methodology used for provincial and federal elections.

Weighting the polls

The vote projection model averages all publicly available opinion polls of decided and leaning voters that meet CBC standards. Polls are weighted by their age and sample size, as well as by the track record and past performance of the polling firm.

Outside of a campaign, the weight of a poll is reduced by about five per cent each day (equating to 35 per cent every 10 days). This weight reduction is incrementally increased once a campaign officially begins until, by the last week of a campaign, the weight of a poll is reduced by 35 per cent each day.

The date of the poll is determined by the last day the poll was in the field, unless the poll was conducted over a period of one week during an election campaign or two weeks outside of a campaign, in which case the date of the poll is considered to be the starting field date plus seven days (or 14 days outside of a campaign).

The sample size weighting is determined by the margin of error that applies to the poll, assuming a completely random sampling of the population.

Pollster track record

Polling firms are weighted by their track record of accuracy over the last 10 years. Their accuracy rating is determined by three factors:

  • The last poll the firm released during an election campaign.
  • The average error for all parties that earned three per cent or more of the popular vote.
  • The amount of time that has passed since the election.

In order to take into account changes of methodology or improvements made over time, the performance of a polling firm in a recent election is weighted more heavily than its performance in an older election.

The difficulty of each election is also taken into account: elections where the average error was smaller are weighted more heavily than elections in which the error was greater. This is meant to take into consideration elections in which there were particular factors contributing to pollster error that were outside of the pollster's control. Conversely, outlier pollsters are penalized more for elections where the consensus was close to the mark.

There is also a reduction in the track record weighting for polls that are outside of the confidence intervals of the vote projection, in order to reduce the extent to which potentially outlying polls make the projection swing unrealistically. This reduction is cumulative, so that a polling firm that is consistently outside of the confidence intervals has its weight increasingly reduced over time. The penalty is removed when a firm's polling is back within the confidence intervals, but returns if the polling results slip outside the confidence intervals again.

The track record rating for a poll that is released without relevant demographic or regional breakdowns is reduced by 50 per cent.

Only polls commissioned by media outlets or independently self-commissioned by polling firms are included in the aggregate. Polls commissioned by political parties, interest groups or advocacy organizations are excluded.

Vote projection

The weightings for sample size, date and track record are combined to give each poll in the projection model an overall weight. No poll is ever awarded more than 66.7 per cent of the total weight, unless there have been no other recent polls. Generally, a limit of 50 per cent is placed on the weight of any one poll.

In short, this means that newer polls with larger sample sizes from polling firms with a good accuracy record are weighted more heavily than older and smaller polls from firms with a poorer track record.

Vote projection ranges

Despite performing better than most individual polls and an unweighted average of all polls, the vote projection is still heavily dependent on the accuracy of the polls. It can fail when the polls do, as occurred in the provincial elections in Alberta in 2012 and British Columbia in 2013.

The projection thus measures the likely error in the vote projection, using the degree of error polls have had in recent elections. The high and low ranges are based on 68 per cent of past election errors and are shown in the confidence ranges in the Poll Tracker's charts.

This means that the result of the election should fall within the high-to-low range 68 per cent of the time.

In one-third of cases, the outcome will fall outside of the projected ranges.

Seat projections

Once the vote projection and likely ranges for each party are determined, the model uses this to make a seat projection.

The seat projection model uses a proportional swing method based on the difference between the results of the last election and current polls. For example, if a party managed 20 per cent in Quebec in the 2019 federal election and is now polling at 40 per cent in Quebec, the party's 2019 election results in each riding in Quebec are doubled.

This swing is applied to every party in each riding. As the result will not always add up to 100 per cent, the projection for each party is adjusted proportionately to equal 100 per cent.

Taking other factors into account

The swing model alone, however, cannot take into account the individual characteristics of each riding. Other factors need to be taken into account.

Incumbent candidates historically have been less sensitive to broader trends than other candidates. When a party is losing support in a region, an incumbent candidate's support drops less than support for other candidates. Conversely, when a party is gaining support in a region, an incumbent candidate receives less of a boost than other candidates.

When a party's incumbent is not running for re-election, its projected vote share is reduced. This penalty is also applied when an MP defeated in the previous election does not run again in the subsequent election.

The presence of a party leader in a riding is taken into account, as leaders have historically been able to outperform other candidates when a party is losing support in a region. Leaders running for the first time in a riding have also historically experienced a significant boost in support.

Similarly to when an incumbent candidate does not run again, there is a steep penalty when a leader vacates a riding, either because they lost it in the previous election or because they are not running for re-election.

Star candidates increase their party's projected vote share by a significant amount. Star candidates are usually former legislators or cabinet ministers, party leaders or well-known figures from the private sector.

If an independent politician is running for re-election, his or her vote is dropped marginally from the previous election, according to the drop in support recorded by independents in other elections. The same penalty is applied to popular independent candidates who were never elected but are running again.

Politicians who left or were forced out of their party caucuses and are running for re-election as independents retain a proportion of their vote share from the previous election, based on the circumstances of their departure from caucus. Those who depart for positive reasons retain much more of their support than those who leave in disgrace. When the circumstances are hard to define, the average performance is used. A no-incumbent penalty is applied to the party the candidate left.

When an ex-legislator attempts a comeback as an independent, using past cases as a guide the independent ex-legislator retains a portion of the vote they received the last time they stood as a candidate.

Byelections are also taken into account when the incumbent party lost the seat or if the results were significantly different from that previous vote (when a major party's support has doubled or been cut in half). The swing from by-elections is calculated by how current polling levels differ from where the parties stood in the polls at the time of the by-election.

When available, and when they differ from the projection's estimations, riding polls are also added to the projection for an individual riding. The weight of the poll is determined by the number of respondents but is capped at 50 per cent of the riding's projection. The riding poll's results are used as a new baseline, from which the numbers are adjusted as regional polling changes. When multiple riding polls are released during an election, only the latest one is taken into account.

When a party is running candidates where it did not have a name on the ballot in the previous election, the riding projection is equal to the regional vote projection for the party.

Likely seat ranges

In order to take into account error in polls and in the seat projection model itself, the vote projection ranges are used to determine likely seat ranges.

For example, if the high projected vote for a party in a given region is five per cent higher than the average projection, then the projected vote for the party in each riding in that region is increased by a factor of 1.05. If the projected high result for a party in a riding is greater than the projected low result for the party expected to win the seat, the seat is then considered winnable for the trailing party.

The seat projection ranges represent all the seats a party could win at the 95 per cent confidence interval — 95 per cent of polling errors should be taken into account in this seat projection range. Seat projections in the tracking chart show the range of seats each party could win at the 68 per cent confidence interval.

The seat projection number itself shows the number of seats in which each party is projected to be ahead. This number will not always be in the centre of the projection ranges. If, for example, a party is leading in a lot of close races, the seat projection will be closer to the top end of the projection range — suggesting that the party has more room to fall than to grow.

Probability of winning

Based on how the seat projection ranges overlap, 10,000 simulations are run to determine the number of cases in which each party can win a majority government or, in the event that no party wins a majority, a plurality of seats. These probability estimates take into account the potential for errors in both the polls and the seat projections to show the likelihood of a specific outcome of an election held on the day of the projection.

To put probability into practical context, a party with a 25 per cent probability of winning has as much of a chance of winning the election as someone does of flipping a coin twice and getting "heads" both times. Unlikely — but still very possible.



Éric Grenier

Politics and polls

Éric Grenier is a senior writer and the CBC's polls analyst. He was the founder of ThreeHundredEight.com and has written for The Globe and Mail, Huffington Post Canada, The Hill Times, Le Devoir, and L’actualité.