Advantages: 

They provide a rich source of qualitative information and they often present an opportunity to research rare disorders or situations otherwise unethical to study. They may also offer some insight on how to conduct further research in the area. 

Limitations: 

They can be invasive and potentially inappropriate, the cannot be generalised to the rest of the population and therefore have low ecological validity, they are very time consuming, they have low reliability because they are nearly impossible to replicate, and they could be subject to investigator bias because the researcher may become attached to the participant. 

*TOP TIP* - The words in bold are great key words you can use to impress the examiner in your essays. 

When you are evaluating research methods, you must talk about them in terms of their ecological validity.