That means we can expect about 12 of these 240 to differ from actual opinion by more than the margin of error. So the September Newsweek poll could have squeezed the trigger on a perfect design but gotten a wild shot by sheer chance. Given these unavoidable limitations, it is generally good advice to average the outcomes of several polls and stick with the pack when reporting the horse-race. But even this approach doesn't always work. PollsterWarren Mitofsky points to the recent Mexican presidential elections, won by Vicente Fox.
Many reporters also forget that in a two-person race, the margin of error applies to each candidate.
3 percent margin of error is 6 percent wide for each candidate. That is, there are two dancing bears inside the box, each with a fuzzy buffer zone 3 percentage points wide on either side of him, left and right. Sometimes they overlap, sometimes they spread apart. It is more reliable to focus on changes in an individual candidate's level of support, rather than the horse-race divide, because it only has one margin of error.