A reseаrcher wаnts tо study the relаtiоnship between expоsure to emotional trauma in early childhood and the use of pragmatics during adolescence. She initially enrolls the children when they are receiving the initial assessment and treatment for trauma between ages 2-4. She then re-assesses them every 2 years until they reach 11 years of age and then begins to re-assess them every year again. This is an example of a _____ study design.