Final answer:
The greatest drawback of the longitudinal method is the extensive time and financial investment it requires, along with high attrition rates.
Step-by-step explanation:
The greatest drawback of the longitudinal method is the extensive time and financial investment it requires. Longitudinal studies often take years, if not decades, to complete and the results are not known until a considerable period of time has passed. Additionally, attrition rates in longitudinal studies are high as participants may drop out or experience life changes that make their continued participation difficult. Researchers must anticipate these challenges and take steps to ensure the sample still represents the larger population.