Which of the following best describes the primary difference between pretest/posttest designs and within-groups designs in developmental research?

• A. Within-groups designs use random assignment, while pretest/posttest designs do not.
• B. Pretest/posttest designs compare different groups before and after an intervention, while within-groups designs measure the same participants under all conditions.
• C. Pretest/posttest designs are only used in longitudinal studies, while within-groups designs are only used in cross-sectional studies.
• D. Pretest/posttest designs do not include any baseline measurements, whereas within-groups designs always include a baseline.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the key terms involved. A pretest/posttest design involves measuring participants before and after an intervention or treatment to observe changes over time.
2. Step 2: Recognize that within-groups designs involve the same participants experiencing all conditions or treatments, allowing comparisons within the same group rather than between different groups.
3. Step 3: Identify that the primary difference lies in how participants are measured: pretest/posttest designs compare measurements at two time points (before and after) often involving different groups or conditions, while within-groups designs measure the same participants across multiple conditions.
4. Step 4: Note that random assignment is not the defining feature separating these designs; both can use random assignment depending on the study setup.
5. Step 5: Conclude that the best description is that pretest/posttest designs compare different groups before and after an intervention, whereas within-groups designs measure the same participants under all conditions.