A longitudinal study is a research method that involves repeated observations of the same subjects over a period of time. This approach allows researchers to track changes and developments in the subjects, which can be individuals, groups, or even entire populations. By collecting data at multiple points, researchers can identify trends, patterns, and causal relationships that might not be evident in a one-time study. These studies are commonly used in fields like psychology, sociology, and medicine. For example, a longitudinal study might follow a group of children from birth to adulthood to examine the effects of nutrition on development. This method provides valuable insights into how various factors influence outcomes over time.