What are the advantages and disadvantages of longitudinal studies?

Advantages are:

1. high in validity - people usually do not remember past events and if they were asked about their past, they would not remember

2. picking up long-term changes

Disadvantages are:

1. it takes a long period of time to gather results

2. a need to have a large sample size and accurate sampling to reach representativness

3. participant may drop out, this is called subject attrition.