Aim: What must we understand before designing an study?

1. Aim: What must we understand before designing an study? HW#2: complete the assignment on the last slide (must be typed and article must be included when submitting)

2. Recap • Sample Surveys are the usual tool for answering questions like “how have attitudes of Americans, on issues ranging from abortion to work, change over time?” • The idea of sampling is to study a part in order to gain information about the whole • Data are often produced by sampling a population of people or things

3. Sample vs Census • A sample is preferred to a census • Census an attempt to contact every individual in the entire population • Not practical • Hard to do • Costly • Timely

4. Why is selecting a sample so important? • If conclusions based on a sample are to be valid for the entire population, a sound design for selecting a sample from the population is needed • You want to generalize results not over-generalize

5. Sample Surveys • Sample surveys collects information about a population by selecting and measuring a sample from the population • The goal is a picture of the population • A sample survey is one type of observational study

6. What is the best way to see the effects of change? • Intervention  can actually impose the change within study • Manipulating independent/explanatory variable • When our goal is to understand cause and effect, experiments are the only source of fully convincing data • Observational data is a poor way to determine what will happen if we change something

7. Class Work: Find a sample survey • Use the internet or some printed material to find an example of a sample survey that interest you. Describe the population how the sample was collected and some of the conclusions • One study of cell phones and the risk of brain cancer looked at a group of 469 people who have brain cancer. The investigators matched each cancer patient with a person of the same sex, age, and race who did not have brain cancer, then asked about use of cell phones. Results: “Our data suggests that use of handheld cellular telephones is not associated with risk of brain cancer.” Is this an observational study or an experiment? Why? What are the explanatory and the response variables? • A typical hour of prime-time television shows three to five violent acts. Linking family interviews and police records shows a clear association between time spent watching TV as a child and later aggressive behaviors. (a) Explain why this is an observational study rather than an experiment. What are the explanatory and response variables? (b) Suggest several lurking variables describing a child’s home life that may be related to how much TV he or she watches. Explain why this makes it difficult to conclude that more TV causes more aggressive behaviors.

8. Homework • Select a newspaper or magazine article that involves a statistical study and write a paper answering these questions. • Is this study descriptive or inferential? Explain your answer. • What are the variables used in the study? In your opinion, what level of measurement was used to obtain the data from the variables? • Does this article define the population? If so, how is it defined? If not, how could it be defined? • Does the article state the sample size and how the sample was obtained? If so, determine the size of the sample and explain how it was selected. If not, suggest a way it could have been obtained. • Explain in your own wordswhat procedure (survey, comparison of groups, etc.) might have been used to determine the study’s conclusions. • Do you agree or disagree with the conclusions? State your reason.