Final answer:
Longitudinal research is complicated by high rates of attrition, which refers to the reduction in the number of research participants due to dropouts. Researchers recruit a large number of participants to account for attrition.
Step-by-step explanation:
Longitudinal research is complicated by high rates of attrition. Attrition refers to the reduction in the number of research participants due to dropouts over the course of a project. This is problematic because participants may discontinue their participation for various reasons such as moving, getting married, or experiencing health issues. Researchers typically recruit a large number of participants to account for the expected attrition.