What is a within-subjects experimental design?

A within-subjects design is an experimental paradigm where all participants receive every level of the treatment or independent variable. For example, in a taste test, each participant would rate every candy type. This approach reduces error from natural individual differences compared to between-subjects designs, where different groups experience different conditions.

Why is counterbalancing important in repeated-measures experiments?

Counterbalancing systematically ensures all possible orders of conditions occur in the study to minimize order effects. Without it, participants would likely perform worse on later conditions due to fatigue or practice effects. By randomizing condition order across participants, researchers prevent these systematic biases from influencing results.

What is an operational definition and why is it needed?

An operational definition is an unambiguous, measurable description of a variable for experimental purposes. It translates abstract concepts into concrete, observable actions. For instance, effort might be defined as the number of weight-lifting curls completed in 30 seconds, making the variable testable and replicable across participants.

How do researchers address ethical concerns in within-subjects experiments?

Researchers provide informed consent describing the study's purpose, procedures, potential risks and benefits, and participants' right to withdraw. Deception may be used when necessary—for example, withholding hypotheses to ensure natural behavior. Debriefing afterward explains the study's true aims and justifies any deception used.

Why do psychological studies require larger sample sizes than other sciences?

Human behavior is inherently variable, so larger sample sizes help ensure the population is better represented and reduce measurement error. Psychological studies typically use higher participant numbers to address this variability. For example, this motivational messaging study used 72 participants to generate reliable, generalizable results.

How are within-subjects designs applied in brain imaging research?

Within-subjects designs are particularly common in functional magnetic resonance imaging (fMRI) research, where participants experience multiple conditions while brain activity is measured. For example, researchers showed participants images of acquaintances, friends, unfamiliar people, and romantic partners to identify brain regions associated with romantic love and reward processing.

What steps should researchers follow when conducting a within-subjects experiment?

First, create operational definitions for all variables. Next, determine all possible condition orders and randomly select one for each participant using counterbalancing. Provide informed consent, run each condition identically except for the independent variable manipulation, record measurements, and debrief participants about the study's true purpose and any deception used.

Within-subjects Repeated-measures Design

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1. Define key variables.

Create an operational definition (i.e., a clear description of exactly what a researcher means by a concept) of a motivational message.
1. For the purposes of this experiment, a motivational message is any combination of image and phrase designed to energize a person’s behavior, manipulated here by viewing a series of images accompanied by empowering quotes focusing on one of 4 areas: hard work, self-affirmation, outcomes/success, and general positive feelings/emotions (Figures 1 - 4).
  
  Figure 1: The image shown with hard work motivational message.
  
  Figure 2: The image shown with self-affirmation motivational message.
  
  Figure 3: The image shown with outcomes/success motivational message.
  
  Figure 4: The image shown with general positive feelings and emotions motivational message.
Create an operational definition (i.e., a clear description of exactly what a researcher means by a concept) of effort.
1. For purposes of this experiment, effort is defined as the participant’s willingness to exert physical strength on a weight-lifting task.

2. Conduct the study.

Meet the student/participant at the lab.
Provide participants with informed consent, a brief description of the research (influences on physical behavior), a sense of the procedure, an indication of potential risks/benefits, the right of withdrawal at any time, and a manner to get help if they experience discomfort.
Determine the order of conditions through counterbalance.
1. It is important to address order effects. If conditions were always in the same order, the later conditions would likely perform worse because participants would be tired.
2. Counterbalanced conditions involve a systematic approach in which the researcher insures that every order occurs during the study. Each condition occurs the same number of times in each of the spots in the order.
3. Each participant receives only one order.
4. Determine all possible orders of the four conditions (Figure 5).
  
  Figure 5: Possible orders for the four conditions.
5. Put each of the 24 possible orders on a slip of paper and place all slips in a bowl.
6. Researcher selects one slip and proceeds to run the experiment in that order. This action randomly selects one of the counterbalanced orders. The researcher should not replace the order in the bowl so that every order gets done once before repeating any one order a second time.
Run the conditions. (NOTE: the researcher will run all 4 conditions using these exact same steps. The only difference will be the image the participant views. The same participant will be shown doing all 4 conditions.)
1. Show the participant a page-sized printout of the image/quote while the participant sits at a table.
2. Tell the participant: “Please read this over and take 1 min to reflect on what it means to you.”
3. After a minute say: “Please stand and take this 10 lb dumbbell in your dominant hand. Complete as many curls as you’d like to in the next 30 s.” Demonstrate the curl motion to the participant. Count aloud as you complete each one.
4. Note the participant’s number of curls on a data sheet (Figure 6).
  
  Figure 6: An example for recording participants’ data by condition.

3. Debrief the participant.

Tell the participant the nature of the study.
1. “Thank you for participating. In this study, I was trying to determine if different types of motivational messages would increase the amount of physical effort participants were willing to exert. There were four types of messages: one emphasizing hard work, one emphasizing what a good person you are, one emphasizing successful outcomes, and one that was generally positive. We hypothesized that the message emphasizing hard work would result in exerting more physical effort.”
Explain explicitly why deception was necessary for the experiment.
1. “We could not tell you about our hypotheses ahead of time because we wanted you to act as naturally as possible.”