time-lag design
a type of quasi-experimental research in which participants of the same age are compared at different time periods. It is typically used in developmental, educational, and social psychological contexts to study whether there are differences in a given characteristic for samples of equal age but drawn from different cohorts measured at different times. For example, a time-lag study of intelligence might compare a group of people who were 20 years old in 2005 with groups who were 20 years old in 2006, 2007, and 2008. Time-lag designs have the benefit of controlling for time-of-testing effects. However, there are drawbacks of low internal validity and difficulty in separating cohort effects from age effects.