-63 a victim, the possibility to dig deep into the causes of the violations, the possibility to explore temporal dynamics of violations, and the possibility to look into the behavioural patterns of other stakeholders (administrators, guards, lawyers, medical professionals, and relatives) etc. In the course of the interview, the respondent can direct to other evidence – detention conditions to observe, victims to interview, documents to see etc. We will deal separately with interviewing detainees in a situation of vulnerability in Section 3.3.5 below. Before that, it is important to underline that NPMs are able to interview a much broader pool of stakeholders, who can provide specific and valuable information on the treatment of the detainees. These include, in addition to the detainees themselves and the staff of the institution, also lawyers, service providers, representatives of religious denominations, relatives of detainees and non-governmental organizations dedicated to the rights of specific vulnerable groups. The choice of the appropriate method to monitor the treatment of people in a situation of vulnerability in detention depends on the specific circumstances of each group. Factors such as placement arrangements (separate or mixed institutions or facilities), the social stigma attached to the particular group, official recognition of the specific vulnerability, access to treatment and services, religious affiliation of the detainees, and the possibility to establish proper communication with them should all be taken into consideration in choosing the method. In any case, the monitoring team should use triangulation, i.e. it should ensure validation of the data collected through cross verification from two or more sources, and by using a variety of research methods. While no monitoring method should be a priori excluded as inappropriate, interviews with prospective victims should be a must with all groups and persons in a situation of vulnerability. NPMs should apply a systemic approach to monitoring vulnerability in detention. This approach takes into account factors influencing human rights developments as a whole, and integrates multiple perspectives in the assessment of the specific situation. It tries to establish patterns and look at systematic failures, not just at the conduct of specific persons. When it tries to explore the root causes of human rights violations, the systemic approach looks not only at deficits, but also at the resources. It Prisoners in a Situation of Vulnerability