Misunderstanding the Margin of Error

From Holden:

Some of our readers may not have a firm understanding of what confidience levels and margins of error mean in the presidential polls I cite. If you are among those who don’t quite get it, you are in good company.

This pamphlet from the American Statistical Association can explain it better than I, but I’m going to give it a shot.

If a poll with a 95% confidence level and a 3% margin of error says Bush leads Kerry 47% to 44%, what it is really telling you is that the pollsters are 95% sure that each candidate’s actual number lies within 3 points of the stated number.

In other words, they are 95% sure that Bush’s number is somewhere between 44% and 50% while Kerry’s number is anywhere from 41% to 47%. Therefore, a poll like my example (Bush 47%, Kery 44% with a 95% confidence level and a margin of error of +-3%) is truly a dead heat.

Here is another explanation that may be easier to understand.

UPDATE: Seems there is also some confusion about what a “dead heat” means. Good thing they have dictionaries.

dead heat n.

1) Sports. A race in which two or more contestants compete evenly or finish at the same time.

2) A political campaign or other contest that is so close that it is impossible to predict the winner.