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April 11, 2011

By David Coletto
The Hill Times

OTTAWA—It’s election time again and we know one thing for certain: polls will be a plenty. They will tell us which parties are up and which are down, who we like and dislike, and what issues we care most about. However, at the beginning of last week, three polls were released that reported very different results.

First, there was the Nanos poll for CTV/Globe and Mail that had the Conservatives with a 14-point lead over the Liberals. Then a Léger Marketing poll found a smaller lead, only 11 points. Then later Monday afternoon Harris-Decima said there was only a seven-point spread between the parties. Either the Tories are headed to a majority, or we are likely to get another minority Parliament. What gives?

Although it’s tempting, it is generally not a good idea to compare polls from different polling firms.

During an election, polling is far more of an art than a science. Each polling firm surveys the country a little differently and this variation can produce different results.

Firms use different data collection methods and different ballot questions. They are in the field on different days, weigh the data in their own way, and each may place the voting question in a different order within the questionnaire. All of these choices can produce what we in the business call “house differences.”

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Good Decisions Require Good Data.