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Introduction to Confidence Intervals :: Introduction to the Margin of Error

Even if you aren't familiar with confidence intervals, you've probably unknowingly run across them. You've probably heard the term "Margin of Error" used along with the results of a survey of, say, a presidential poll.

After polling 1000 eligible voters, the Star-Tribune Newspaper reported that 55% of Americans would vote for James Bean and 45% for John F Daniels +/- 3%.

That plus or minus disclaimer is the margin of error. In other words, the margin of error means that James Bean could be favored by as much as 58 to 42 percent (55 + 3) or as low as 52 to 48 percent (55 - 3)-- a six percentage point spread (58-52 = 6). This spread is the confidence interval

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