Transcript 4.1 Statistics Notes

4.1 Statistics Notes
Should We Experiment or Should
We Merely Observe?
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)
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?
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
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
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
Homework
 10/28 HW#23 p154 2,4,7,8,11,12