The study of a person, a small group, a single situation, or a specific "case," is called a case study. It involves extensive research, including documented evidence of a particular issue or situation -- symptoms, reactions, affects of certain stimuli, and the conclusion reached following the study. A case study may show a correlation between two factors, whether or not a causal relationship can also be proven. It may sound complicated, but it's really quite simple.
For example, a case study may show that obese people tend to drink a lot of diet soda. This would mean that there is a correlation, or association, between being overweight and drinking diet soda. No causal factor has been shown, or in other words, there is no scientific evidence that indicates drinking diet soda actually causes obesity, just that there is an association between the two.
Another case study may prove both causal and correlative factors. If research and evidence show that obese people also tend to eat more fattening foods than leaner people, there is science to back up the fact that eating fat causes people to gain weight. The difference between causal and correlative factors is important in the study, because sometimes such studies are used to promote a specific medicine, therapy, or product; or to show that a particular product is unhealthy, unsafe, or should be used with care.
A case study may be used to show that a medicine is safe for the largest percentage of a certain demographic, based on the physical evidence, interviews, and observations. On the other hand, the same resources may show that the medicine is unsafe for certain segments of the population, including certain age groups. A case study may also be used to test other products or services, or even to decide which business model is most cost efficient.
A case study can be an important tool for establishing the effectiveness and safety of a product. Depending on who is conducting the study, a company can also use it as a tool to discount claims made by competing manufacturers. To better decipher case studies, be sure to pay close attention to the factors examined, and ascertain whether they are correlative, causal, or both.