A recent poll putting the Liberals and Conservatives at roughly even strength in Alberta, but with a 10% margin of error got me to thinking about the accuracy of such polls, and what a 10% margin of error means. It certainly sounds quite high, but does it mean that the poll is useless? I wanted to know.
So I whipped up a spreadsheet in Excel to simulate polls. 100 people polled represents a 10% margin of error, so that's the sample size I simulated. In Alberta, there's an approximate support level of about 20% for parties other than the Liberals and Conservatives, so I set that and consequently started at 40% Liberal and 40% Conservative "true" support. I ran 20000 simulated polls in each test with varying "true" differences in support. These are the results:
So that means that if you poll 100 people, and the true levels of support are 39-41-20 (Con-Lib-Oth for example), my simulation gave the leading party a leading poll result 56.6% of the time. In other words, with a 10% margin of error, a 2% lead in true support was accurately reflected more than half the time. A 4% true lead was accurately reflected 65.6% of the time. a 10% lead in support, a level equal to the margin of error, was accurately reflected 86.2% of the time. The poll inspiring this investigation reported a 40% lead vanishing to nothing. So I simulated a 40% lead too. This lead was NEVER falsely reported, even with a margin of error of 10%.
So, even though the accuracy at low margins is poor, even with a 10% margin of error and a true lead of only 2%, the poll results as far as showing who's leading and who's trailing are probably correct.
Just something to think about whenever you read poll results.
CKA Super Elite 
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