Answer:
A longitudinal design is a systematic observation that involves obtaining measures of the variables of interest in multiple waves over time.
Step-by-step explanation:
A longitudinal design is a type of research that seeks to observe the behavior of one or more variables through time, it is helpfull in the understanding of change and variation. In order to do so, systematic observation is involved on different occasions over the course of the research.
In a longitudinal design the sample must remain the same during the time of the study, in order to identify changes and variations.