Lesson Establishing Causation

Knowledge Objectives Identify the three ways in which the association between two variables can be explained. Define what is meant by a common response. List five criteria for establishing causation when you cannot conduct a controlled experiment.

Construction Objectives Explain what process provides the best evidence for causation. Define what it means to say that two variables are confounded. Discuss why establishing a cause-and-effect relationship through experimentation is not always possible. Explain what it means to say that a lack of evidence for a cause-and-effect relationship does not necessarily mean that there is no cause-and-effect relationship.

Vocabulary Causation – a direct cause-and-effect link between variables Common Response – variables change in response to changes to a third variable (and not really to changes in each other)

Common Response Changes in x cause changes in y Changes in both x and y are caused by another variable z The effect of x on y is confounded by the other variable z # of ticks and cases of spotted fever # of ticks, boating accidents, and summer weather

Confounding When two variables influences on a response variable is hard to distinguish from one another, we refer to that a confounding. Confounding variables can be other explanatory variables or a variable not included in the analysis (extraneous) Remember we don’t want to use the term “lurking variable” because of the nuances it carries. Use extraneous variable instead

Why not Experiment Always? The best evidence for causation comes from experiments in which the researcher controls the explanatory variable(s) So why do we do observational studies? –Ethics –Not physically possible

Causation in Absence of Experiment 1.The association is strong studies show very strong correlations 2.The association is consistent studies across wide ranges of different populations or areas show same strong correlations 3.Larger values of the response variable are associated with stronger responses more explanatory variables correspond to more (or less) of the response variable 4.The alleged cause precedes the effect in time the cause has time to effect the response 5.The alleged cause is plausible experiments (where do-able) back the larger effect

Summary and Homework Summary Homework –pg