Longitudinal research involves studying the same sample of individuals over multiple points in time. There are three main types of longitudinal studies: trend studies sample different groups over time from the same population; cohort analyses measure characteristics of groups that experience the same life events; and panel studies measure the same sample at different points to reveal changes. Combining qualitative and quantitative data in longitudinal research allows qualitative data to aid interpretation and suggest new hypotheses. Conducting longitudinal research online through internet surveys panels has advantages like easier recruitment but also disadvantages like lack of representation.