A case study or case history consists of information about an individual’s language learning experience that may be of particular interest. Case histories of good language learners, may be of interest, however they are often outliers and the evidence may be somewhat anecdotal.
It can be difficult to generalise from individual case studies to general population. They can have greater depth than group studies. While it is difficult for a case study to lend support to a hypothesis, it is somewhat easier for a case study to disprove a hypothesis (black swan). Case studies may become known for "being the documented first example of x". They may be easy for student to remember because they involve storytelling.
Several case studies are relevant to second language acquisition; see category:case studies.