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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

Click on the gray buttons to see this relationship in the image below.





 
Home Introduction to the Margin of Error Samples Vary Smaller Samples Vary More than Larger Samples Margin of Error Quiz More Margin of Error The Confidence Level Relationship Between the Confidence Level & Confidence Interval Confidence Level Quiz The Variability of a Sample Variability Affects the Confidence Interval Confidence Interval QuizView All Tutorials
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September 25, 2011 | Ed M. wrote:
Fantastic exposition! I'd been all over the intranets searching for a concise explanation of the concept of margin of error, and everywhere else I just found a bunch of confusing goobledygook. But yours is super clear with a great concrete example. Thanks for this!!
 
June 27, 2011 | Judith Wanstall wrote:
"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). " In the frst portion of the staement I understand where the 58 comes from (55+-3%). Where does the 42 come from? Ahhh the balance of scales. James gets 3% John looses 3%
 
March 30, 2009 | Caryl wrote:
How the minus and plus margin were worded or connected in the last sentance. to show how James Bean was favored over John Daniels. I understand the CI spread, but did not see the connection with plus and minus margins
 
March 15, 2009 | Anthony wrote:
Basic and good explaination.
 
March 13, 2009 | Henry N.Rivera wrote:
Yes, it makes sence.
 
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