Statistics For Dummies®, 2nd Edition

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

Good survey and experiment researchers always include some measure of 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 beginning 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 ...