Statistical Sciences 1024A/B Chapter 9: Chapter 9 - Producing Data- Experiments

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Statistical Sciences
Statistical Sciences 1024A/B
Lori Murray

February 7, 2017 Chapter 9: Producing Data: Experiments 9.1 Observation vs. Experiments In contrast to observational studies, experiments dont just observe individuals or ask them questions. They actively impose some treatment in order to measure the response. An observational study observes individuals and measures variables of interest but does not attempt to influence the responses. The purpose of an observational study is to describe some group or situation. An experiment, on the other hand, deliberately imposes some treatment on individuals to observe their responses. The purpose of an experiment is to study whether the treatment causes a change in the response. When our goal is to understand cause and effect, experiments are the only source of fully convicting data. Two variables (explanatory variables or lurking variables) are confounded when their effects on a response variable cannot be distinguished from each other. Observational studies of the effect of one variable on another often fail because of confounding between the explanatory variable and one or more lurking variables. o Ex: if we simply observe peoples drinking choices, the effect of moderate drinking is confounded with (mixed up with) the characteristics of people who choose to drink in moderation Diet and lifestyle (lurking variables) are confounded with drinking habits (explanatory variable) ** Welldesigned experiments take steps to avoid confounding** 9.2 Subjects, Factors, and Treatments The individual studied in an experiment are often called subjects, particularly when they are people The explanatory variables in an experiment are called variables A treatment is an specific experimental condition applied to subjects. If an experiment has more than one factor, a treatment is a combination of specific values of each factor. Ex: Effect of Advertising o Objective: To determine the effects of repeated exposed to an advertising message (may depend on length and how often repeated) o Subjects: a certain number of undergraduate students o All subjects viewed a 40minute TV program that included ads fro a digital camera. o Some subjects saw a 30 second commercial; others say a 90 second version o Same commercial was shown either 1, 3, or 5 times during the program o 2 factors Length of commercial (2 values) Number of repetitions (3 values) 6 treatments
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