the apparent-time hypothesis depends on several assumptions: first, that a significantly broad sample is taken to be representative of the population; second, that vernacular speech is relatively stable in a given individual once that individual is past adolescence. apparent-time studies are examples of cross-sectional studies, meaning a population group is examined at a certain point in time. within the population, age-stratified variation is assumed to reflect the ages at which members of the population learned language. it is often contrasted with real-time studies, which are examples of longitudinal studies.