Researchers are restricted to using either a between-subjects design or a within-subjects design, but not both, when investigating a single overall research question across a series of different studies.

Using one type of experimental design does not preclude researchers from using the other type. There is no reason a researcher cannot utilize both a between-subjects design and a within-subjects design to answer the same overall research question across a series of different studies. In fact, professional researchers frequently adopt this type of mixed methods approach to gain a more comprehensive understanding.

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A between-subjects experiment is an experimental design approach wherein each participant is exposed to only a single level of the independent variable. This method differs fundamentally from within-subjects designs, as individuals are tested in just one specific condition rather than across all experimental variations.

Between-Subjects Experiment

A within-subjects experiment is an experimental design strategy where every participant is exposed to all levels of the independent variable. This approach contrasts with between-subjects designs by testing the exact same individuals across every experimental condition instead of assigning them to just one.

Within-Subjects Experiment

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KPU Research Methods in Psychology - 4th American Edition

Random assignment is the primary method used to control extraneous variables across experimental conditions. It involves using a chance-based procedure to determine which participants are placed into which conditions. Strictly speaking, it requires that every participant has an equal probability of being placed in any condition and that each individual's assignment is completely independent of the others.

Random Assignment

Between-subjects experiments offer distinct practical benefits compared to within-subjects approaches. They are conceptually simpler to design and demand less testing time per participant. Crucially, because each individual is only exposed to a single experimental condition, this design naturally avoids carryover effects and completely eliminates the need for complex counterbalancing procedures.

Advantages of Between-Subjects Experiments

The independent-samples $$t$$-test is a statistical procedure utilized to determine if there is a significant difference between the means of two distinct groups ($$M_1$$ and $$M_2$$). This analytical method is appropriate when the two samples are tested under varying conditions within a between-subjects experimental design, or when researchers are comparing pre-existing groups (such as extraverts and introverts) in a cross-sectional research design.

Independent-Samples t-Test

The one-way ANOVA is a specific type of analysis of variance used to compare the means of more than two samples ($$M_1, M_2 \ldots M_G$$) to determine if there are statistically significant differences among them. This statistical test is specifically applied within between-subjects designs where a single independent variable is manipulated across multiple independent groups.

One-Way ANOVA

A matched-groups design serves as an alternative to simple random assignment. In this approach, researchers first measure the dependent variable or a relevant extraneous variable to assess the participants. They then rank-order the participants based on this measurement and randomly assign the most closely matched individuals to different experimental conditions. This technique guarantees that the groups are equivalent on that specific variable at the start of the study, preventing it from becoming a confounding variable.

Matched-Groups Design

Examples of between-subjects experiments include randomly assigning a sample of university students to either a traumatic writing condition or a neutral writing condition, ensuring each student only completes one type of task. Similarly, a study evaluating treatments for severe agoraphobia might divide participants into separate groups so that each person receives only one of three possible therapies.

Examples of Between-Subjects Experiments

Birnbaum's 1999 study serves as a classic demonstration of the problems associated with a lack of context in between-subjects designs. When participants rated the number $$9$$ or $$221$$ on a size scale in separate groups, the group rating $$9$$ judged it as mathematically 'larger' than the group rating $$221$$. Without the context of a within-subjects comparison, participants spontaneously compared $$9$$ to other single-digit numbers and $$221$$ to other three-digit numbers, skewing their relative size judgments.

Example of Lack of Context in Between-Subjects Designs

Combining Between-Subjects and Within-Subjects Designs

In a between-subjects experiment, when researchers utilize random assignment to allocate participants to different conditions, the resulting groups are expected to be highly similar to one another. Because these groups lack systematic pre-existing differences, researchers consider them to be equivalent.

Equivalent Groups

When participants in a between-subjects experiment are placed into conditions without the use of random assignment, the resulting groups will likely be dissimilar in various ways. Because these groups possess inherent, pre-existing differences from the outset, researchers consider them to be nonequivalent.

Nonequivalent Groups

Which of the following best describes a between-subjects experiment?

A notable disadvantage of within-subjects experiments is that experiencing all experimental conditions makes it much easier for participants to guess the researcher's hypothesis. This awareness can inadvertently change their behavior; they might judge situations in a way they think is expected of them, or they might try to act similarly across conditions in an unnatural effort to be 'fair'.

Hypothesis Guessing in Within-Subjects Designs

A simultaneous within-subjects design is an approach where, instead of testing participants in one distinct condition at a time, researchers present stimuli from multiple conditions in an interspersed or mixed sequence. Participants make multiple responses to the varying conditions throughout a single session, and the researcher then calculates each participant's mean response or overall performance for each distinct condition.

Simultaneous Within-Subjects Design

An order effect is a primary disadvantage of within-subjects experimental designs. It occurs when participants' responses in various experimental conditions are systematically affected by the order in which they were exposed to those conditions. When order effects are not the direct focus of the research, they can act as problematic confounding variables that obscure the true effect of the independent variable.

Order Effect

When deciding between experimental approaches, researchers generally prioritize the within-subjects design if practical. A good rule of thumb is to opt for a within-subjects experiment—utilizing proper counterbalancing—if there is sufficient time to test each participant in all conditions and if there are no serious concerns about confounding carryover effects. If these conditions cannot be met, researchers should consider a between-subjects design instead.

Rule of Thumb for Experimental Design Choice

Within-subjects experiments offer distinct benefits by naturally controlling for extraneous participant variables. Because the same individuals are tested in every condition, researchers can reduce statistical noise, making it easier to detect any true effect of the independent variable upon the dependent variable. Consequently, these designs also require fewer overall participants than between-subjects experiments to detect an effect of the same size.

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