-48 Often, the information that leads to the monitoring of certain aspects of solitary confinement comes from isolated detainees, whom the NPM meets, even if at the time of the visit they are no longer isolated. In such cases, it is a good practice to initially ask the prison administration for a list of all detainees and their location within the institution. Afterwards the NPM can interview the detainee who sent the information along with many others, so that the choice of interviewees appears random in order to avoid the risk of retaliation. Another good practice is to request the internal regulation regarding the isolation section(s), if there is one. During the visit, it is good practice for NPMs to access all places in the facility, also those which look unused. Spaces have to be investigated not only if they are used, but also if they are empty, in order to understand why they are empty, and if they might be places where informal isolation might take place. 4.3.2. Prison records The first step to identify any critical issues related to isolation consists of the examination of prison records/registers on sections, critical events, disciplinary measures, medical records (collective or individual medical records), and the individual file of the detainee along with his plan to leave a situation of isolation and reintegrate him/her in the general prison population (if there is one). To also have an overview of the past situation of the penitentiary institute, many NPMs examine a sample of past cases of prisoners who were isolated. Some examine the latest ones or those who were isolated for the longest time, others all those from the previous year. The study of the records reveals the average duration of the isolation measures. In the case of disciplinary solitary confinement, this makes it possible to measure the gap between the average length of the measure and the maximum duration set by the law (in many cases, prison administrations decide to give the maximum duration provided by the law without following any criteria of proportionality). The records make it possible to identify the