A<unk>  study is a correlational research study that involves repeated observations of the same items over long periods of time — often many decades.  It is a type of observational study.<unk>  are often used in psychology to study developmental trends across the life span, and in sociology to study life events throughout lifetimes or generations. The reason for this is that unlike cross-sectional<unk> ,<unk>  track the same people, and therefore the differences observed in those people are less likely to be the result of cultural differences across generations. Because of this benefit,<unk>  make observing changes more accurate and they are applied in various other fields.  In medicine, the design is used to uncover predictors of certain diseases.  In advertising, the Communicus System, the design is used to identify the changes that advertising has produced in the attitudes and behaviors of those within the target audience who have seen the advertising campaign.
Because<unk>  are observational, in the sense that they observe the state of the world without manipulating it, it has been argued that they may have less power to detect causal relationships than do experiments. But because of the repeated observation at the individual level, they have more power than cross-sectional observational<unk> , by virtue of being able to exclude time-invariant unobserved individual differences, and by virtue of observing the temporal order of events.<unk>  allow social scientists to distinguish short from long-term phenomena, such as poverty. If the poverty rate is 10% at a point in time, this may mean that 10% of the population are always poor, or that the whole population experiences poverty for 10% of the time. It is not possible to conclude which of these possibilities is the case using one-off cross-sectional<unk> .
Types of<unk>  include cohort<unk>  and panel<unk> . Cohort<unk>  sample a cohort, defined as a group experiencing some event (typically birth) in a selected time period, and studying them at intervals through time. Panel<unk>  sample a cross-section, and survey it at (usually regular) intervals.
A retrospective study is a<unk>  study that looks back in time. For instance a researcher may look up the medical records of previous years to look for a trend.
Longitudinal studies
Longitudinal study
Placebo-controlled studies
Clinical trial
Cohort study
Randomized controlled trial
Cross-sectional study
Action study