Longitudinal studies follow a particular group over a period of time. A test or observation is repeated over the study which usually lasts a long time and allows all the data to be gathered. For example, to study the effects of age a researcher might study a group of children when they are 4 and then again when they are 16. They use either the same method of data collection or take a similar approach to case studies and use many different methods.
The EPPE project and NICHD study are two examples of longitudinal studies in child psychology and the case study of Genie is also an example, which uses just one child instead of many.
Studying the same people throughout allows for control over participant variables like gender, temperament and IQ. They are a really good way for studying development because a controlled procedure will then lead to the possibility of cause-and-effect conclusions being drawn which is a lot easier than the different people used in cross-sectional studies.
Many factors will still affect the development of the people be looked at. It can still be hard to draw conclusions about one feature which might cause change. Also as they take place over a long period of time there is sometimes a high drop-out rate because people move away or no longer want to take part – these people might have something in common and therefore the sample may become biased (everyone who stays the whole way through may be quite confident or from a certain type of family).