Lecture 4 Notes: Internal Validity and Experimental Design
Internal Validity in Experiments
Introduction to Internal Validity
Focus of the lecture: Understanding how experiments establish internal validity when other methods do not.
Importance of ruling out variables besides the independent variable to confirm causal relationships.
Causation and Experimental Design
Essential factors for establishing causation in experiments:
Covariance: Achieved by having a statistically significant difference in means.
Temporal Precedence: Established as the independent variable precedes the dependent variable.
Internal Validity: Focus of discussion; ensured through random assignment, which minimizes alternative explanations.
Role of Random Assignment
Random assignment eliminates alternative explanations and third variables, unlike multiple regression which can only rule out some third variables.
Well-designed experiments leave no room for third variables impacting the outcome; they establish a causal link between variables, although the word "prove" should not be used in scientific discussions.
Importance of understanding that experiments establish causal links rather than proof of causation.
Common Experimental Designs
Pharmaceutical Research Example:
Participants receive either a real drug or a placebo.
Depression scores are measured after the treatment period to determine efficacy of the medication.
Ensures that if random assignment is properly conducted and no significant internal validity threats exist, conclusions can reflect a causal relationship.
Experimental Control Paradigm:
Clinical Psychology Example: Comparing a new therapeutic technique against a waitlist group as a control condition.
Cognitive Psychology Example: Examining effects of background noise on attention through a controlled environment.
Generally, psychology tends towards comparison approaches rather than strictly control approaches.
Comparison vs Control Conditions
Comparison Group: One that receives some treatment or intervention but not the same intensity or type as the experimental group.
Example: Comparing a new therapy to existing therapies instead of a waitlist or no treatment.
At least one comparison or control group is necessary; lacking either makes results uninterpretable.
One Sample Experiments: Rare situations where a single group can be tested, typically found in specific statistical conditions, such as one-sample t-tests.
Experimental Methods in Psychology
Science is defined as a method rather than specific disciplines like chemistry or biology.
Example: Acidic vs. basic liquids demonstrated through litmus paper, illustrating how controlled conditions can yield reliable conclusions.
Clinical Psychology Example: Assessing the effectiveness of wilderness therapy against antidepressants with randomly assigned samples.
Interpreting Results
Concerns about interpreting results when only small numbers of participants are used in each condition.
Example comparing two individuals receiving different treatments; intuitive doubts about causation despite adherence to statistical principles, highlighting people's tendency to view individual behavior variability differently than chemical/physical experiments.
Group Equivalence: Random assignment maintains comparable groups in regards to significant variables.
Individuals maintain characteristic variability, but random assignment averages these differences across conditions.
Addressing Individual Differences
Random assignment isn’t a box to tick; it is an effective method for ensuring equal distribution of participant characteristics.
The necessity of having an adequately large sample size ensures that results can be generalized and trustworthy.
Random Assignment vs. Random Sampling
Random Assignment: Allocates participants to conditions within the experiment to control for internal validity.
Random Sampling: Ensures diversity in the sample and external validity of results.
Distinction is crucial; random assignment addresses internal validity while random sampling addresses generalizability.
Examples Illustrating Random Assignment
A memory experiment example comparing studying pictures versus words shows how random assignment negates potential influencing factors related to individual aptitudes for memory.
Random assignment enables the researcher to focus solely on the treatment effect and eliminates potential confounders.
Conclusion and Next Steps
In subsequent lectures, alternatives to random assignment, such as matching and using within-subject designs, will be discussed.