The areas of consideration in any case study need to take into account the variables which could effect the results.
Sampling is a particularly significant factor.
The sample you use- whether is be people, or something less tangible, for example if you were studying different trends, or a particular geographical area- will be the thing that provides you with your results, so you need to make sure you choose wisely.
Outside influences
it is almost impossible to conduct any kind of research which is entirely unaffected by outside influences, but you need to ensure that these are kept to a minimum, or that you have a way of accounting for them.
Ethical you must consider the ethical implications of either conducting your research, or what influence your findings may have. Or both.