Chapter 4: Designing Studies Section 4.1 Samples and Surveys The Practice of Statistics, 4th edition – For AP* STARNES, YATES, MOORE

Random = Good. Each student should take an SRS of 20 chips from the bag. Record the proportion of chips that are red. Return the chips to the bag. Make a dotplot of the class’s results.

Trust Why can we trust samples that are random? The results obey the laws of probability. Eventually, we’ll be able to say things like, “We are 95% confident that…” For now, one point is worth noting: LARGER RANDOM SAMPLES GIVE BETTER INFORMATION ABOUT THE POPULATION THAN SMALLER SAMPLES.

Sample Surveys: What Can Go Wrong? Most sample surveys are affected by errors in addition to sampling variability. Good sampling technique includes the art of reducing all sources of error. Sampling and Surveys Definition Undercoverage occurs when some groups in the population are left out of the process of choosing the sample. Nonresponse occurs when an individual chosen for the sample can’t be contacted or refuses to participate. A systematic pattern of incorrect responses in a sample survey leads to response bias. The wording of questions is the most important influence on the answers given to a sample survey.

Sampling errors vs. Non-sampling errors A sampling error is a mistake made in the process of taking a sample that could lead to inaccurate information about the population. Undercoverage is a sampling error. Undercoverage occurs when the “sampling frame” doesn’t list every member of the population. Poor sample designs lead to sampling errors. These are errors that don’t stem from the sampling process – they could even result from a census! Examples of non-sampling errors: Nonresponse Response bias Wording of questions

Do you brush your teeth at least twice a day and floss every day? News flash: most people don’t answer this question honestly.

Two Questions – wording matters! Should illegal immigrants be prosecuted and deported for being in the U.S. illegally, or shouldn’t they? 69% favored deportation. Should illegal immigrants who have worked in the United States for two years be given a chance to keep their jobs and eventually apply for legal status? 62% said yes.