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Between-Subjects Design: Examples, Pros & Cons

By Julia Simkus, published Feb 15, 2022


A between-subjects study design, also called independent-groups, is one way that researchers can assign test participants to different treatment groups.

In a between-subjects design, each subject is assigned to only one treatment condition, and researchers will compare group differences between participants in these various conditions.

Using a between-subjects design

In a between-subjects design, there is usually a control group and an experimental group with each participant experiencing one of these conditions. These study designs can have multiple treatment conditions, though, so a study with three conditions.

For example, would have three groups of subjects, with each group receiving one of the three treatment conditions. To prevent bias, the participants should be randomly assigned to either the control group or one of the experimental conditions, and they should not know which group they are assigned.

The goal of a between-subjects study design is to enable researchers to determine if one treatment condition is superior to another. Researchers will manipulate an independent variable to create at least two treatment conditions and then compare the measures of the dependent variable between groups.

They will measure whether the groups differ significantly from each other due to the different levels of the treatment variable that they experienced.

This method is called between-subjects because the differences in conditions occur between the groups of subjects. A between-subjects design is the opposite of a within-subjects design, where each participant experiences every condition and the differences in the conditions happen within a given subject across conditions.

For example, assume a psychiatrist is looking for new medication to treat patients with Obsessive-Compulsive Disorder (OCD). She has four potential options of medications to help the patients with their OCD. In order to determine which medication is going to be the most beneficial for her patients, she creates four testing groups among her population of patients.

Each group receives one of the four medications. Researchers then analyze these patients and collect data to test their anxiety levels. The psychiatrist can use this study to decide which medication is best for her patients with OCD.

Advantages

Avoid carryover effects

Carryover effects between conditions can threaten the internal validity of a study. A carryover effect is an effect of being tested in one condition on participants' behavior in later conditions.

For example, exposure to a reaction time test could make participants’ reactions times faster in a subsequent treatment if the same subjects were participating in both conditions.

However, in between-subjects study designs, the participants are divided into different treatment groups, so one participants’ exposure to a treatment will not affect the outcome of a subsequent condition.

Short and straightforward

Each participant is only assigned to one treatment group, so the experiments tend to be uncomplicated. Scheduling the testing groups is simple, and researchers tend to be able to receive and analyze the data quickly.

Limitations

Large participant pool is necessary

Because each subject is assigned to only one condition, this type of design requires a large sample. Thus, these studies also require more resources and budgeting to recruit participants and administer the experiments.

Individual differences

Differences between subjects within a given condition may be an explanation for results, introducing error and making the effects of an experimental condition less accurate.

Examples

  • Resistance to extinction of human evaluative conditioning using a between‐subjects design (Baeyens, Diaz, & Ruiz, 2011).
  • Startle reflex modulation by pleasant and unpleasant odors in a between-subjects design (Ehrlichman et al., 2007).
  • Faking self-reports of health behavior: a comparison between a within- and a between-subjects design (Egele, Kiefer, & Stark, 2021).
  • The Effects of a Fatal Vision Goggles Intervention on Middle School Aged Children's Attitudes toward Drinking and Driving and Texting and Driving as Related to Impulsivity: A Between Subjects Design (Carey, Lester, & Valencia, 2016).
  • How to show that 9 > 221: Collect judgments in a between-subjects design (Birnbaum, 1999).
  • The Impact of the 2018 North Korea-United States Summit on South Koreans’ Altruism Toward and Trust in North Korean Refugees: Between-Subjects Design Around the Summit (Chang and Kang, 2018).

Frequently asked questions about between-subjects design

1. What's the difference between a within-subjects versus between-subjects design?

Between-subjects and within-subjects designs are two different methods for researchers to assign test participants to different treatments.

In a between-subjects design, researchers will assign each subject to only one treatment condition; whereas in a within-subjects design, researchers will test the same participants repeatedly across all conditions.

Between-subjects and within-subjects designs can be used in place of each other, or in conjunction with each other.

Each of these types of experimental design has its own advantages and disadvantages, and it is usually up to the researchers to determine which method will be more beneficial for their study.

2. Can you use a between-subjects and within-subjects design in the same study?

Yes. Between-subjects and within-subjects designs can be combined in a single study when you have two or more independent variables (a factorial design).

Factorial designs are a type of experiment where multiple independent variables are tested. Each level of one independent variable (a factor) is combined with each level of every other independent variable to produce different conditions.

Each combination becomes a condition in the experiment. In a factorial experiment, the researcher has to decide for each independent variable whether to use a between-subjects design or a within-subjects design.

In a mixed factorial design, researchers will manipulate one independent variable between subjects and another within subjects.

About the Author

Julia Simkus is an undergraduate student at Princeton University, majoring in Psychology. She plans to pursue a PhD in Clinical Psychology upon graduation from Princeton in 2023. Julia has co-authored two journal articles, one titled “Substance Use Disorders and Behavioral Addictions During the COVID-19 Pandemic and COVID-19-Related Restrictions," which was published in Frontiers in Psychiatry in April 2021 and the other titled “Food Addiction: Latest Insights on the Clinical Implications," to be published in Handbook of Substance Misuse and Addictions: From Biology to Public Health in early 2022.

How to reference this article:

Simkus, J. (2022, Feb 15). Between-Subjects Design: Examples, Pros & Cons. Simply Psychology. www.simplypsychology.org/between-subjects-design.html

Sources

Allen, M. (2017). The sage encyclopedia of communication research methods (Vols. 1-4). Thousand Oaks, CA: SAGE Publications, Inc doi: 10.4135/9781483381411

Baeyens, F., Díaz, E., & Ruiz, G. (2005). Resistance to extinction of human evaluative conditioning using a between‐subjects design. Cognition & Emotion, 19(2), 245-268.

Birnbaum, M. H. (1999). How to show that 9> 221: Collect judgments in a between-subjects design. Psychological Methods, 4(3), 243.

Carey, A. A., Lester, T. G., & Valencia, R. M. (2016). The Effects of a Fatal Vision Goggles Intervention on Middle School Aged Children's Attitudes toward Drinking and Driving and Texting and Driving as Related to Impulsivity: A Between Subjects Design (Doctoral dissertation, Brenau University).

Chang, H. I., & Kang, W. C. (2018). The Impact of the 2018 North Korea-United States Summit on South Koreans’ Altruism Toward and Trust in North Korean Refugees: Between-Subjects Design Around the Summit. Available at SSRN 3270334.

Egele, V. S., Kiefer, L. H., & Stark, R. (2021). Faking self-reports of health behavior: a comparison between a within-and a between-subjects design. Health psychology and behavioral medicine, 9(1), 895-916.

Ehrlichman, H., Brown Kuhl, S., Zhu, J., & WRRENBURG, S. (1997). Startle reflex modulation by pleasant and unpleasant odors in a between‐subjects design. Psychophysiology, 34(6), 726-729.

Jhangiani, R. S., Chiang, I.-C. A., Cuttler, C., & Leighton, D. C. (2019, August 1). Experimental Design. Research Methods in Psychology. Retrieved from https://kpu.pressbooks.pub/psychmethods4e/

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