1. Causation & Experimental Design
Research Methods
Chapter 6
Causation & Experimental Design

2. Three Criteria For Determining Causation in a Hypothesis
Correlation (association)
Time Order
Non-Spuriousness
Two additional things used to strengthen a causal statement
Identify Causal Mechanism
Specify the context

3. Why Experiment?: True Experiments
True experiments must:
have at least 2 comparison groups
Experimental Group
Control Group
have variation in the Ind. Variable before assessment of the dependent variable
Have subjects randomly assigned to 2 or more groups
Ensures internal validity but not generalizability
Sometimes matching is used as a poor substitute for random assignment

4. Validity in Experiments
Exp. Are well suited to produce valid conclusions about causality
Less well suited to achieve generalizability

5. Let’s take a look at some of the ways in which experiments help (or don’t help) to resolve potential problems with causal (internal) validity and generalizability…

6. What Are The Threats to Validity in Experiments?
Threats to causal (internal) validity
Non-comparable groups
When the experimental group and control group are not really comparable
Endogenous change
When natural developments in the subjects account for changes between pre and post tests
History
When subjects become aware of things outside the experiment (like the news) that affects their outcome scores
Contamination
When comparison and treatment groups somehow affect one another
Treatment Misidentification
When subjects experience a treatment that was unintended
Generalizability (The Achilles heel of true experiments….)
Sample Generalizability
Cross-Population Generalizability
Interaction of Testing and Treatment