OCT 18 Experimental Designs study

Experimental Designs Overview

Importance of Understanding Experimental Designs

  • Understanding how to design experiments is crucial in psychological research.

  • Experimental designs allow researchers to investigate causal relationships between variables.

Key Questions Addressed

  • How do confounding variables affect the internal validity of an experiment?

  • What are the three major steps of a basic experiment?

  • What is an independent group design?

  • What is a repeated measures design?

  • What is a matched pairs design?

Internal Validity and Confounding Variables

Definition of Confounding Variables

  • Confounding variables DEF: variables that are intertwined with another variable so that you cannot determine which of the variables is operating in a given situation.

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  • They prevent the determination of the true cause-and-effect relationship.

  • Cannot determine whether IV or confounding variable is responsible for the effect

Internal Validity

  • Internal validity DEF: refers to the extent that a study can meet all 3 criteria for cause and effect

  • Cause and effect criteria :

    1. can establish temporal precedence

    2. can establish covariation

    3. eliminate most alternative explanations

  • Covariation DEF: the principle that changes in one variable (the cause) are associated with changes in another variable (the effect). In simpler terms, covariation occurs when there’s a consistent relationship between two variables, such that when one variable changes, the other also changes in a predictable way.

  • Temporal precedence DEF: This is the concept that the cause must occur before the effect in time. For a cause-and-effect relationship to be established, the cause (independent variable) has to happen before any changes in the effect (dependent variable) can be observed. Temporal precedence helps in asserting that the cause actually led to the effect and didn’t just happen simultaneously or afterward.

  • High internal validity is achieved by ensuring the IV is the only factor affecting the dependent variable (DV).

Steps of a Basic Experiment

Three Major Steps in Experimental Design

  1. Obtain Two Equivalent Groups

    • Participants are divided into two groups that are comparable.

    • Groups should be equivalent before manipulation of the IV.

  2. Introduce the Independent Variable

    • The IV is manipulated to observe its effect on the DV.

  3. Measure the Effect of the Independent Variable

    • The DV is measured after the manipulation to gauge the impact of the IV.

Types of Experimental Designs

Independent Groups Design

  • Participants are assigned to different levels of the IV.

  • Each participant experiences only one condition.

  • Random assignment ensures groups are equivalent.

Repeated Measures Design

  • Same participants are used in all conditions of the experiment.

  • Reduces variability among participants.

  • Example: Measuring the same group of people's responses before and after an event.

Matched Pairs Design

  • Participants are matched in pairs based on certain characteristics (e.g., age, gender).

  • One member of each pair is assigned to one condition, and the other member is assigned to the alternative condition.

Pretest-Posttest Designs

Overview

  • Involves measuring the DV before and after the introduction of the IV.

  • Variants of this design include the Solomon four-group design:

    • Half of participants receive both pretest and posttest.

    • Half receive only the posttest.

Advantages

  • Helps maintain equivalency in small samples.

  • Useful when dropout is expected or participants are unique to specific conditions.

Disadvantages

  • Can be time-consuming and may sensitize participants to the research goals.

  • Results may not generalize well if the pretest effect is significant.

Solutions to Disadvantages

  • Use deception to mask the study's true intentions.

  • Embed pretest measures among irrelevant tasks to reduce sensitivity.

Importance of Random Assignment

  • Random assignment is crucial to ensure that any differences observed in the DV are indeed due to the IV.

Next Steps for Students

  • Finalize project proposal form and ethics checklist by October 18.

  • Submit all relevant documents and the team evaluation form by October 22.

  • Maintain communication with the instructor via email for any queries.