Detailed, in-depth, longitudinal anaylsis of an individual group or event. They tend to take place over a long period of time and can involve gathering data from family or friends.
They often involve analysis of unusual individuals or people with rare disorders. The data collected is mainly qualitative.
+)Provides rich, detailed, insight into the specific person. Such detail increases the validity of the data collected.
-)Small sample makes it difficult to make generalisations. Low external validity.