How the NDP Won the 2015 Alberta Election
Posted by Sridhar Mutyala at 11:46 AM · No Comments

After 44 years of PC rule, Albertans voted in a majority NDP government in Tuesday’s election. The result surprised many even though polls were predicting an NDP landslide. Wherever you fit on the political spectrum, until it happened, it didn’t seem possible.

So how did it happen? The fall (or death) of the PC dynasty. Rachel Notley and the impression she made on Albertans. Naheed Nenshi nudging Calgary voters to be open to alternatives. Prominent businessmen hectoring Edmontonians to stick with the status quo. Alberta’s changing demographics. Or how about mirrors, downturns, hope, fear, or math?

During the campaign, Jim Prentice dismissed the possibility of Albertans electing an NDP government, because, in his words, Alberta is not an NDP province. In the election aftermath, some suggested that, once you got past voter anger, the reason for the big NDP win was the province’s gradual shift to the left.

So does an NDP majority mean that Alberta is now an NDP province? Or is there some other explanation available in the voting data? Let’s start with a list of the NDP’s seats and compare the party’s share of the popular vote to the combined share of the PC and Wildrose parties.


The NDP had strong support in Edmonton and some surrounding communities as well as Lethbridge and Calgary Fort. But its remaining seats, in Calgary and other parts of central and northern Alberta, were won with a very slim margin in ridings where the majority of voters chose the parties on the right.

Support for the NDP clearly surged in this election, but where did the support come from? From a fired up base? From new voters? From voters switching from other parties? Or from habitual non-voters who decided to cast a ballot this time around?

To explore these questions, we built a mathematical model for the riding results data. We assumed that voters could be split into groups based on the party they’re most likely to support or align with. We also assumed that voters in each group don’t unconditionally vote for their aligned party; they could vote for other parties. Finally, we assumed that different ridings are made up of a mixture of these different voter groups.

We divided the voter population into 5 groups, 4 groups for voters aligned to each of the 4 major parties and 1 group for everyone else. Our objective function was the difference between the model’s predicted vote results and the actual vote results for each party in each riding. We defined 2 constraints: the probabilities assigned to parties in a group must sum to 1, and the group proportions in a riding must sum to 1. We then used the scipy optimization module to find the party preferences for each group and the group proportions for each riding that minimized the objective function.

The table below shows the party preferences (rows) for the 5 voting groups (columns) in the 2015 election. The numbers in each column represent the probability that the voters in the group will select the given political party. The percentage at the bottom of each column is the group’s proportion of the total voter population.

Voting groups in 2015:


The first group of voters preferred the Widrose party. Their probability of choosing the Wildrose was 43%, the PC party 14%, and the NDP 9%. They were 33% likely to not vote, and they made up 30% of the voting population.

The NDP group wasn’t the biggest, but it was the most likely to vote and the least likely to seek alternatives. The party also drew support from all the other groups, even the Wildrose. The NDP was even the choice of the non-voter group when it decided to vote.

So, a fired up base, support from other camps, and folks coming in from the sidelines plus other party supporters more likely to stay home and less likely to stay loyal — that’s the NDP story in 2015.

Voting groups in 2012:


The NDP group grew from 9% of the voter population in 2012 to 21% in 2015. This didn’t happen at the expense of the two conservative groups. Their total share remained about the same between the two elections (around 47%; voters migrated from the PC to the Wildrose group but stayed in the conservative fold).

The new NDP voters seemed to have come primarily from the non-voter group (which saw its share decline by 9%). This suggests the party courted and converted previous non-voters and increased its viability with other voter groups to expand its constituency. (Check out how this data-driven analysis lines up with what the party leaders had to say about their election strategy.)

Both these trends should concern conservatives. The NDP has gotten much better at political life, and Albertans like and trust the party’s leader. What should concern them more still is how homogeneous the PC and Wildrose groups really are (check the second and third choice parties for each group) and what this means for the unite the right movement.

In response to Jim Prentice, Rachel Notley said that “Albertans will decide what sort of province Alberta is.” Alberta may not be an NDP province yet (most Alberta voters are still in the Wildrose and PC camps), but you can sort of see it from here.

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