A/HRC/8/3 page 28 precise policy on how the various criteria interact to determine who should be detained together or apart. Thus, as a purely hypothetical example, one might separate inmates into age, sex and other groups required by law; further segregate each such group by the severity of the offence committed; then, among those responsible for violent crimes, separate persons from rival gangs; and finally separate out leaders of gangs. The system of classification and segregation that is required will vary according to the particular challenges facing each prison system, but too often Governments allow these decisions to be made on an ad hoc basis by individual officials; instead, they should be clearly spelled out and made known to all concerned. Second, the institutional means to implement this classification and segregation policy must be put in place. To effectively screen and sort new inmates, there will be a need for staff trained in interviewing new inmates and in reaching out to other law enforcement authorities to obtain and analyse information on the criminal histories and gang affiliations of individuals and on the relationships between gangs. Third, the policy must be continuously evaluated for its effectiveness in, inter alia, preventing prisoner-on-prisoner violence, the establishment of gang control and recidivism. 79. This brief discussion of one tool - developing a system for allocating prisoners to cells, wings and prisons - that Governments have for ending the hold of prisoner authorities and preventing prisoner-on-prisoner violence should not be taken to imply that preventing violent gangs from controlling prisons is straightforward. In any such effort, unintended consequences are common. Providing vulnerable prisoners with protective custody in response to gang threats can perpetuate gang control. Attempting to isolate gang leaders from the general population can spark violent riots. However, for all the difficulties of re-establishing Government control, it is clear that the necessary tools are available. 80. Even when prison officials do make serious and sensible efforts to prevent violence and to assert their legitimate disciplinary authority, reform may prove elusive unless certain underlying factors are addressed. Prisons run by prisoners are typically also characterized by understaffing, overcrowding and corruption. Governments that are serious about maintaining the monopoly of violence, which is a function belonging solely to the State, must address these underlying problems in relation to the use of violence in prisons. 81. Understaffing, or an insufficient ratio of staff to prisoners, makes it difficult and often dangerous for staff to supervise inmates effectively. In extreme cases, a small staff has no choice but to prioritize searching visitors and securing the perimeter against escapes while leaving inmates almost entirely unsupervised. However, even in more typical situations, understaffing increases the temptation to engage in corruption and to exercise power indirectly through prisoners. 82. Overcrowding makes it much more difficult to prevent prisoner-on-prisoner violence. Cells are difficult to monitor as effectively as common areas, and this inherent danger is made worse by the tendency for competition for space among a cell’s inmates to lead to violence. Overcrowding also makes it more difficult to take other preventive measures. Even in the rare situation in which overcrowding does not lead directly to an insufficient staff-to-prisoner ratio, the direct supervision of inmates is dangerous in a densely packed area. In addition, overcrowding can make it difficult or impossible to find the space for programming or to effectively classify and segregate inmates.