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

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

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

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

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…

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