Longitudinal study

When longitudinal studies 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 experiments. Others say that because of the repeated observation at the individual level, they have more power than cross-sectional observational studies, by virtue of being able to exclude time-invariant unobserved individual differences and also of observing the temporal order of events. Longitudinal studies do not require large numbers of participants (as in the examples below). Qualitative longitudinal studies may include only a handful of participants, and longitudinal pilot or feasibility studies often have fewer than 100 participants. == Disadvantages ==

Longitudinal studies are time-consuming and expensive. Longitudinal studies cannot avoid an attrition effect: that is, some subjects cannot continue to participate in the study for various reasons. Under longitudinal research methods, the reduction in the research sample will bias the remaining smaller sample. Practice effect is also one of the problems: longitudinal studies tend to be influenced because subjects repeat the same procedure many times (potentially introducing autocorrelation), and this may cause their performance to improve or deteriorate. ==Examples==