Page 197 - Statistics for Dummies

Chapter 12
Leaving Room for a Margin of Error
In This Chapter
▶ Understanding and calculating margin of error
▶ Exploring the effect of sample size
▶ Finding out what margin of error doesn’t measure
ood survey and experiment researchers always include some measure
Gof how accurate their results are so that consumers of the information
can put the results into perspective. This measure is called the margin of
error (MOE) — it’s a measure of how close the sample statistic (one number
that summarizes the sample) is expected to be to the population parameter
being studied. (A population parameter is one number that summarizes the
population. Find out more about statistics and parameters in Chapter 4.)
Thankfully, many journalists are also realizing the importance of the MOE in
assessing information, so reports that include the margin of error are begin-
ning to appear in the media. But what does the margin of error really mean,
and does it tell the whole story?
This chapter looks at the margin of error and what it can and can’t do to help
you assess the accuracy of statistical information. It also examines the issue
of sample size; you may be surprised at how small a sample can be used to
get a good handle on the pulse of America — or the world — if the research
is done correctly.

Seeing the Importance

of That Plus or Minus

Margin of error is probably not a new term to you. You’ve probably heard of
it before, most likely in the context of survey results. For example, you may
have heard someone report, “This survey had a margin of error of plus or
minus three percentage points.” And you may have wondered what you’re
supposed to do with that information and how important it really is. The
truth is, the survey results themselves (with no MOE) are only a measure of

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