Case studies are a type of qualitative research method. Identify a topic that would be studied best by using this method. Be specific as to what one can learn from this study. This question is related to Health Services Research Methods.
The term "case" in a case study is used in much the same way as it might be in detective work; it refers to a particular occasion or series of events involving specific people and places. The case study observes the subjects of the case, their circumstances and history, and attempts to explain the relationship between the two. If this seems like a very broad approach to take, it is; case studies are essentially a method of generalizing data from widely disparate sources. For this reason, case studies cannot always produce data that can reliably be followed to a change in methods. A common reminder is that "correlation does not equal causation" - for example, a higher rate of obesity may not necessarily be caused by a shared socioeconomic background, but by shared genes, or by poor eating habits.
A single case study is usually insufficient to build a reputable database. Without a wide range of studies to compare to, there is no way of knowing if the data of a particular case represents common or outlier points. Furthermore, the aforementioned problem of correlation may only reveal its mistakes on a broader scale; for example, if it is found that a difference of ten pounds correlates to groups that do and do not drink soda, this data would be useful, but it would take time to reliably collect it, or reliably identify the correlation, especially if it is an obscure or unintuitive one.
A good case study would often prefer to select the most "deviant" circumstances. The reasoning for this is straightforward; highly divergent cases must be based on highly divergent causes. If these causes can be traced and identified, their importance and relationship to each other can be identified, predicted, and observed in other groups. Average groups are necessary to provide good data, but the variances between them will be very small; thus it will be difficult to observe exactly what the causes of these minor differences are because there is so much overlap in the circumstances and reasons that most of us encounter.
Consider, using the obesity example, the difference between studying a group that is all slightly overweight (by 5 pounds or so) and a group of people who are more than 200 pounds overweight. The +200 pound group will provide substantial information about the lifestyle, psychology, diet, physical health and environment of the group, whereas the 5 pound group may be that way merely because of an extra donut or two in the morning every day.
A good case study might be severely overweight people who do not make efforts to lower their weight or change their eating habits, and instead advocate "health at every size" philosophy which exclaims that it is possible to be healthy at extremely high weights. This stands in contrast to medical science, and has overlaps into social behaviors like bullying and cognitive dissonance. A case study might examine how these people came to weigh so much, what allows them to perpetuate that weight, and the ways in which they guard themselves from criticism. The study may provide us with a knowledge of when and how the transition to obesity occurred, and how defensive psychological mechanisms and self-esteem can maintain a positive body image in the face of widespread social criticism.