Book T of C

Chap T of C

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This is the 2007 version. Click here for the 2017 chapter 01 table of contents.

Assuming you *do* have a truly random sample, the accuracy of the sample (for purposes of representing the larger population) can be calculated and expressed as *sampling error*. Sampling error is the amount of error one might expect, based upon the size of a random sample.

What is the "margin of error"?

You have probably seen sampling error reported in poll results. For example, a poll might show that candidate A has the support of 43% of the population "plus or minus 3%." The plus or minus 3% is the sampling error, often called the *margin of error*. A poll result of "43% plus or minus 3%" usually means, "With 90% likelihood the *true* figure for the *entire* population is between 40% and 46%." The concept of sampling error is based on the ideal of a random sample. In a truly random sample, *everybody in the target population has an equal chance to be included in the sample*. As long as that condition is met, scientists can calculate the margin of error or expected accuracy of the sample.

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