Which issue arises in designs where participants experience several conditions and the order may influence outcomes?

• A. Order effects
• B. React tivity effects
• C. Placebo group
• D. Baseline measure

Answer: (A)

Order effects happen when the sequence in which someone experiences multiple conditions changes their responses. In designs where the same participants go through several conditions, what they do or feel in an earlier condition can carry over, improve with practice, or tire them for later tasks. This can make differences between conditions reflect the order rather than the actual manipulation, threatening the study’s internal validity. For example, completing a task once might teach a strategy that helps in subsequent conditions, or fatigue after an initial condition might reduce performance later—even if the later condition itself is no harder.

This concern is especially important in repeated-measures or crossover designs where each participant encounters all conditions. Mitigation strategies include counterbalancing or randomizing the order of conditions so that any order-related effects are spread across conditions rather than biasing a particular comparison.

The other concepts don’t fit this specific issue. Reactivity effects arise when participants change behavior because they know they’re being studied. A placebo group concerns controlling for expectancy or nonspecific treatment effects. A baseline measure is simply the initial assessment used for comparison.