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Sep. 4, 2007 - News

How to Estimate the Margin of Error of a Random Poll

Pollsters often report a margin of error along with their data for random polls. For instance, in the latest Fox News/Opinion Dynamics Poll, conducted Aug. 21-22, 314 Republican voters were polled at random nationwide as to which candidate they would vote for in the Republican presidential primary. The reported margin of error (MoE) is 5.5%. How was this figure obtained and what does it mean? There is a simple formula for computing the MoE from the number N of people polled. The formula is

MoE = 100%/sqrt(N)

which in this case is 100%/sqrt(314) = 5.6%. The difference between this figure and the reported figure of 5.5% is probably due to the poll not being entirely random and is insignificant in any case.

So how are we to interpret the margin of error? Consider the reported percentage of Republican voters polled who said they would vote for Rudy Giuliani. This figure is 29%. Since the MoE is 5.5%, this means that the true percentage of ALL Republican voters nationwide who would say they would vote for Rudy Giuliani is very likely to be between 23.5% and 34.5%, since 23.5% is one MoE less than the reported percentage of 29% and 34.5% is one MoE more than 34.5%. How likely is it that the true percentage? Statisticians typically report a 95% confindence interval, meaning that there is a 95% chance that the true percentage of Republican voters nationwide who say they will vote for Rudy Giuliani is between 23.5% and 34.5%.

Democratic presidential hopeful Hillary Clinton is out to prove that this is her election to win, not her husband former president Bill clinton. For more information go to afp.google.com.

Democratic hopeful Barack Obama believes Hillary Clinton is all talk and hasn't said exactly how she would make a real changed in our nation's direction. For more information go to forbes.com.

Republican presidential hopeful Rudy Giuliani may be the class of the Republican Party so far but he is not exactly the favorite in the presidential election. For more information go to abcnews.go.com.

Republican candidate John McCain is set to take the underdog role in the presidential election but that doesn't change his opinion that he is a strong candidate. For more information go to washingtonpost.com.

Republican hopeful Mitt Romney believes his campaign is already strong and only going to keep getting even stronger as the election nears. For more information go to washingtonpost.com.