Longitudinal effects refer to the impact of variables or treatments over an extended period of time. In research, longitudinal studies involve collecting data from the same group of individuals or subjects at multiple points in time to observe changes or trends over time. This type of study allows researchers to track the effects of various factors on the same group of individuals over an extended period, providing valuable insights into how variables interact and influence outcomes over time. Longitudinal effects are particularly useful for understanding patterns of change, development, and stability in fields such as psychology, sociology, medicine, and public health.