3.7 Experimental designs | Quantitative methods | Research Designs | UvA

Summary

This script discusses the importance of true experiments for maximizing internal validity in research. It outlines key elements such as manipulation of the independent variable, comparison between conditions, and random assignment. The video introduces four common experimental designs: the two-group design, the two-group pre-test post-test design, the Solomon four-group design, and the repeated measures design. Each design is tailored to address potential threats to internal validity, such as maturation and pre-test sensitization, ensuring robust conclusions can be drawn from the study.

Takeaways

• 🔬 A true experiment is essential for maximizing internal validity in research.
• 🧬 Key elements of a true experiment include manipulation of an independent variable, comparison between different conditions, and random assignment to these conditions.
• 📝 Four common experimental designs are discussed: two-group, two-group pre-test post-test, Solomon four group, and repeated measures.
• 🎲 The two-group design involves random assignment to an experimental and control condition, with the independent variable differing between them.
• 📊 The two-group pre-test post-test design adds a pre-assessment of the dependent variable to account for initial differences and potential maturation effects.
• 🚫 The pre-test can sometimes alert participants to the study's purpose or cause a practice effect, impacting internal validity.
• 🤔 The Solomon four group design combines pre-test and no pre-test conditions to account for potential pre-test effects.
• 🔄 The repeated measures design involves participants experiencing all levels of the independent variable, with random assignment to the order of conditions.
• 📐 This design controls for individual differences by using the same subjects in all conditions, but can introduce order effects.
• 📈 The effectiveness of a pre-test can be assessed by comparing results from groups with and without it, informing the design of follow-up studies.
• 📚 The transcript emphasizes the importance of considering various threats to internal validity and the use of different designs to mitigate them.

Q & A

What is a true experiment and why is it important for maximizing internal validity?

A true experiment is a research design that includes manipulation of the independent variable, comparison between conditions with different levels of the independent variable, and random assignment to these conditions. It is important for maximizing internal validity because it helps ensure that any observed effects are due to the independent variable rather than extraneous factors.

What are the key elements of a true experiment?

The key elements of a true experiment are manipulation of the independent variable, comparison between conditions exposed to different levels of the independent variable, and random assignment of participants to these conditions.

What is the simplest experimental design mentioned in the script?

The simplest experimental design mentioned is the two-group design, where participants are randomly assigned to an experimental condition and a control condition.

What is the purpose of the experimental condition in a two-group design?

The purpose of the experimental condition is to introduce the hypothesized cause or independent variable to observe its effect on the dependent variable.

What is the purpose of the control condition in a two-group design?

The control condition serves as a baseline to compare against the experimental condition, where the hypothesized cause is absent, allowing for the assessment of the independent variable's effect.

What is a two-group pre-test post-test design and why is it used?

A two-group pre-test post-test design adds a pretest of the dependent variable before exposure to the independent variable. It is used to check for initial equality between groups and to control for maturation or other threats to internal validity.

What is a potential issue with using a pretest in an experiment?

A pretest can sometimes sensitize participants, leading to a practice effect or alerting them to the study's purpose, which may negatively affect internal validity.

What is the Solomon four group design and how does it address the issues with pretests?

The Solomon four group design is a combination of the two-group design and the two-group pre-test post-test design, run twice—once with a pretest and once without. It helps to account for the unwanted effects of pretests by comparing the results of groups with and without pretests.

What is the repeated measures design and how does it differ from between-subjects designs?

The repeated measures design involves one within-subjects factor where all participants are exposed to all levels of the independent variable. Unlike between-subjects designs, random assignment is not to the conditions themselves but to the order in which the conditions are experienced.

Why might a researcher choose to use a repeated measures design?

A researcher might choose a repeated measures design to control for individual differences between participants, as each participant serves as their own control by experiencing all conditions.

How does the order of conditions in a repeated measures design affect the study?

The order of conditions in a repeated measures design can affect the results due to factors like learning effects, fatigue, or other order-related biases. Random assignment to the order helps to mitigate these effects.