Dr. Kang studies the development of morality in children. In her latest study she followed and assessed 3year-olds, 4-year-olds, and 5-year-olds for a period of time. The 3-year-olds were tested at ages 3, 4, and 5; the 4-year-olds at 4, 5, and 6; and the 5-year-olds at ages 5, 6, and 7. This is an example of a ___________ research design. correlational cross-sequential longitudinal cross-sectional