Cross-sectional research involves using ‘cohorts’ of different groups of people who differ in one key characteristic (the variable of research interest) at one specific point in time but share other characteristics, such as socioeconomic status, educational background, and ethnicity. For example, researchers might create cohorts of participants who differ in age (say, are in their 30s, 40s, and 50s). The given question refers to cohorts of seven and nine year olds.