1. 4.1 Statistics Notes Should We Experiment or Should We Merely Observe?

2. Types of Studies  A researcher conducts an experiment by assigning subjects to certain experimental conditions, called treatments, and then observing outcomes on the response variable  In an observational study, the researcher observes values of the response and explanatory variables (without imposing any treatments)

3. Cause and Effect  Establishing a cause and effect relationship is central to science. Unfortunately, we already learned that association and correlation cannot imply causation.  In an observational study, lurking variables can affect our results.  An experiment reduces the potential for lurking variables. Why?

4. Advantages of Experiments over Observational Studies  In an experiment with a RANDOM selection to determine the subjects that receive the treatments, the different treatment groups tend to be balanced on all other variables, including lurking variables. Therefore, the lurking variable will not affect any associations.  Example: FDA approval  No random assignments can be made in an observational study. Therefore lurking variables cannot be accounted for in observational studies.  We can study the cause and effect relationship of explanatory variables on response variables more accurately using experiments than observational studies

5. Why Use Observational Studies?  There are ethical reasons why experiments cannot be performed and only observations can be made.  For example, say we want to find out if cell phone usage causes cancer. Can we randomly assign some people to use a cell phone every day which may cause them to get cancer, and tell others they can never use a cell phone at all?  Some questions do not involve assessing causality. Therefore observational studies can be used.  Ex. A public opinion poll; elections

6. Sample Surveys  A sample survey selects a sample of people from a population and interviews them to collect data  A sample survey is a type of observational study  For a sample survey to be informative, it is important that the sample is representative of the population  A census is a type of sample survey that attempts to sample the entire population

7. Homework  10/28 HW#23 p154 2,4,7,8,11,12