Month: February 2007

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The magical sample size in polls

gshmueli

Now that political polls are a hot item, it is time to unveil the mysterious sentence that accompanies many public opinion polls (not only political) — This typically reads “the poll included 1033 adults and has a sampling error of plus or minus three percentage points”. No matter what population is being sampled, the sample size is typically around 1,000 and the precision is almost always “ۭ±3%” (this is called the margin of error). If you type “poll” in Google you will find plenty of examples. One example is the Jan 2, 2007 NYT Business section article “Investors Greet New … Continue reading The magical sample size in polls

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The legendary threshold of 5% for p-values

gshmueli

Almost every introductory course in statistics gets to a point where the concept of the p-value is introduced. This is a tough concept and usually takes time to absorb. It is also usually one of the hardest concepts for students to internalize. An interesting paper by Hubbard and Armstrong discuss the confusion in marketing research which takes place in textbooks and journal articles. Another “fact” that usually accompanies the p-value concept is the 5% threshold. One typically learns to compare the p-value (that is computed from the data) to a 5% threshold, and if it is below that threshold, then … Continue reading The legendary threshold of 5% for p-values