-61 combine both types of methods in their monitoring. The advantages of the quantitative methods are in the possibility to obtain statistical and representative information from a larger number of detainees, and to better ensure the anonymity of the respondents. Their disadvantages are in the very basic and general type of the information obtained, and the reduced likelihood of going into details of the particular circumstances of the individual respondent. Qualitative methods allow much better possibility to dig deep into the circumstances of each case. 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. Quantitative methods seem to be underused by NPMs nowadays and there are no good reasons for this. Nothing should prevent an NPM to conduct a representative survey in a specific detention facility, or in the system of the criminal detention as a whole. The experience of those organizations who have carried out such surveys in the past are very positive and the results have proven extremely valuable. In 2016-2017, the Bulgarian Helsinki Committee carried out a survey among 1,357 convicted prisoners in all the prisons of Bulgaria, whose pre-trial proceedings had started after 1 January 2015. According to the survey findings, every third person (34%) surveyed reported being physically ill-treated either upon police arrest or in police custody. Those claiming use of physical force within police custody were more (24%) than those who reported violence upon arrest (19.4%). The number of inmates claiming physical illtreatment at investigation detention facilities was significantly lower – 6.2% of those who had been detained at such facilities. In general, the survey did not reveal any positive dynamics in the trends, and even estimated a slight increase of the use of unlawful physical force over the past several years. The survey results revealed that two groups are particularly in a situation of vulnerability visà-vis physical abuse by the police – Roma and juvenile prisoners. The share of Roma (28.3%) who reported being victims of physical abuse by police officers was twice as high as that of Bulgarians (14.5%). Prisoners in a Situation of Vulnerability