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Introduction to Confidence Intervals :: Samples Vary

Of course, the Star-Tribune Newspaper didn't ask everyone in the US who they were going to vote for; instead, they sampled a portion of the total voting population. Since most polls only sample a portion of a larger population, another sample of 1000 people might provide slightly different results. Instead of 55 to 45, another sample might provide 54 to 46. The margin of error is a way to describe this variability.




This same principle applies in many settings. Consider another example:
Cash Fast, a manufacturer of ATM machines, hired a research firm to find out how long it took people to withdraw money. This firm watched 20 people go to an ATM and withdraw money. They kept track of how long each person took and reported an average time of 2 minutes +/- 30 seconds.

There's our friend the Margin of Error. It can be used whenever samples are taken and an estimate is made about a larger population. Did you also notice that the margin of error is half the confidence interval? So an easy way to know the confidence interval if all you know is the margin of error is to multiply the margin of error times two.

So remember the confidence interval = 2 times the margin of error.


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