Two teams study vocabulary growth. Team X gives a vocabulary test to 200 children aged 4, 6, 8, and 10 in 2025 and compares scores. Team Y gives the same test to 200 four-year-olds in 2025 and retests the same children every two years until 2031. Both teams report age-related vocabulary gains.
A major threat to the validity of Team Y's findings is:
- A
Lack of repeated measurement
- B
Confounding age with birth cohort
- Ccheck_circle
Attrition that may leave a non-representative subsample
- D
Inability to detect age differences
Explanation
Longitudinal studies lose participants over time; if dropout is non-random (for example, lower-performing children leave more often), the remaining sample becomes biased. Cohort confounding is a problem for cross-sectional, not longitudinal, designs.