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.
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In a between-subjects design, there is usually a control group and an experimental group, with each participant experiencing one of these conditions. However, these study designs can have multiple treatment conditions, so a study with three conditions.
For example, there would be three groups of subjects, each 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. They should not know which group they are assigned to.
A between-subjects study design aims 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. The differences in the conditions happen within a given subject across conditions.
For example, assume a psychiatrist is looking for a new medication to treat patients with Obsessive-Compulsive Disorder (OCD). She has four potential options for medications to help 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.
Avoids carryover effect
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’ reaction times faster in a subsequent treatment if the same subjects participated in both conditions.
However, in between-subjects study designs, the participants are divided into different treatment groups, so one participant’s exposure to 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.
A 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.
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.
- Resistance to the 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
1. What’s the difference between a within-subjects versus a 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. In contrast, 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 type 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.
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/