approach to vulnerability in places of interviewing, detention. Many legal systems lack and case studies. Quantitative methods definitions of vulnerability. EU member (statistical states themselves do not collect or research, cluster analysis, surveys) are publish data on all groups in a situation much less common. of vulnerability and their share of the total number of prisoners. Some NPMs try to define vulnerability on the basis of the circumstances arising within the specific national context. This is a healthy approach, which should be encouraged. analysis of analysis, complaints correlational However, in order to explore the root causes of the problems which the affected individuals and groups face, there is often a need to use both qualitative and quantitative methods. NPMs are in a good position to combine both methods and adopt a systemic approach to their monitoring activities and their follow-up. Monitoring the rights of detainees Interviewing in a situation of vulnerability poses of vulnerability in detention requires a variety of challenges. The OPCAT specific approaches and skills. NPMs takes into account such challenges by should make use of the best practices providing for some specific powers of of interviewing groups in a situation of the NPMs in Article 20 and by shielding vulnerability in social research, taking those who supply information to them into from repression in Article 21. Without vulnerability and the need to ensure their doubt, monitoring should be based on safety. There should be no compromise the established principles and methods with the imperatives of personal security of social research. The leading principle of the detainees, even at the expense of among these is the “Do No Harm” the quality of the research. principle. Other principles include accuracy, impartiality, confidentiality, transparency and addressing vulnerability. So far, detention monitors at the international and at the national level used mostly qualitative methods in their monitoring work, including observation, persons consideration in their a situation continuous