What is the main advantage of a within-subjects design over a between-subjects design?

What is the main advantage of a within-subjects design over a between-subjects design?

• A. Requires larger sample sizes
• B. Reduces external validity
• C. More power
• D. Increases random assignment requirements

Answer: (C)

When the same participants experience every condition, individual differences between people are held constant across conditions. This reduces the variance in scores that comes from who the participant is, leaving more of the observed differences to be explained by the experimental manipulation. In statistical terms, the error variance drops, which increases the test’s power to detect a true effect. That higher power is the primary practical benefit, often allowing researchers to use fewer participants while still achieving the same sensitivity. The other statements aren’t advantages: needing larger samples is not a benefit of within-subjects designs; they can actually require fewer participants. External validity isn’t inherently improved and can even be compromised in some within-subjects setups due to repeated testing or limited contexts. Random assignment requirements are usually reduced because each participant serves in all conditions, so the design relies less on assigning people to different groups.

When the same participants experience every condition, individual differences between people are held constant across conditions. This reduces the variance in scores that comes from who the participant is, leaving more of the observed differences to be explained by the experimental manipulation. In statistical terms, the error variance drops, which increases the test’s power to detect a true effect. That higher power is the primary practical benefit, often allowing researchers to use fewer participants while still achieving the same sensitivity.

The other statements aren’t advantages: needing larger samples is not a benefit of within-subjects designs; they can actually require fewer participants. External validity isn’t inherently improved and can even be compromised in some within-subjects setups due to repeated testing or limited contexts. Random assignment requirements are usually reduced because each participant serves in all conditions, so the design relies less on assigning people to different groups.