PSYC 217 03c three types of experimental designs

Review and Overview of Experimental Designs

  • Goal of session: Compare and contrast three basic experimental designs essential for the course.

    • Designs focus on controlling for participant variables to avoid confounding issues.

Importance of Controlling Participant Variables

  • Participant variables can confound results, hindering confident interpretations of study outcomes.

  • If participant variables are not controlled, subsequent experiments may be required to determine causal relationships.

  • Three designs to control participant variables:

    1. Between Subjects Design

    2. Matched Pairs Design

    3. Within Subjects Design

Between Subjects Design

  • Definition: Participants are randomly assigned to one condition only.

    • Notation: Denoted as "SS" for subjects/participants.

  • Example scenario: Assigning groups based on birth dates for a word color game.

    • Group 1: Birth dates from January to June.

    • Group 2: Birth dates from July to December.

  • Task: Participants verbally state the color of displayed words, ignoring the word text itself.

  • Explanation of potential confounds in assignment:

    • Group assignment based on birth dates may correlate with age and cognitive differences, influencing performance.

  • Outcomes represented in bar charts comparing mean scores for each group.

Matched Pairs Design

  • Definition: Participants paired based on a pretest variable that may influence the dependent variable (DV).

    • This design ensures balance in the characteristics being controlled for.

  • Procedures:

    • Measure the pretest variable (e.g., perceptual processing speed) before the main experiment.

    • Participants are matched and assigned to different conditions (e.g., matching vs. mismatching).

  • Visualization of Results:

    • Bar charts centered at zero, indicating performance related to pretest speeds.

    • Bar heights above or below zero indicate comparative speeds during the actual testing versus pretest performance.

  • Advantages:

    • Useful for small sample sizes where random assignment may fail.

    • Helpful in costly studies requiring participant preparation (e.g., fMRI, EEG).

    • Mitigates dropout issues in longitudinal studies.

  • Limitations:

    • Pretests may influence psychological experiences, potentially skewing results (e.g., practice effects during matching/mismatching tasks).

Solomon Four Group Design

  • Purpose: To assess the influence of a pretest on study results.

  • Structure: Involves four groups:

    1. Pretest + Matching

    2. Pretest + Mismatching

    3. No Pretest + Matching

    4. No Pretest + Mismatching

  • This design allows researchers to analyze the effect of pretest exposure on performance.

  • Key outcomes reveal if pretest contributes to behavior changes, guiding further research methodology decisions.

Within Subjects Design (Repeated Measures Design)

  • Definition: Each participant experiences all levels of the independent variable (IV).

  • Advantages:

    • Greater statistical power due to fewer resources expended (only need one group of participants).

    • Eliminates participant variable confounds because the same individual responds in all conditions.

  • Limitations:

    • Subject to order effects where initial exposure influences subsequent trials.

    • Example: Practice effects lead to improved performance in later trials unrelated to manipulation of IV.

Comparison of Experimental Designs

  • Repeated Measures:

    • Ideal for achieving higher statistical power, requiring fewer participants than between subjects.

    • Limitations include the challenge of controlling order effects.

  • Matched Pairs:

    • Balances individual differences among small groups but risks pretest altering performance.

  • Between Subjects:

    • Higher participant count needed; random assignment is essential to ensure validity but can lead to other potential confounds if not properly managed.

Conclusion

  • Decision on design depends on specific research questions, the sensitivity of participants to different tasks, and the concept being studied.

  • Insightful observation and feedback from participants are essential in adjusting designs for accuracy in experimental research.

  • Importance of ethical considerations, minimizing bias, and understanding participant behavior throughout the research process.

  • Thanks attendees for participation and anticipation of future learning opportunities in the course.