-45 a much larger part is ‘hidden under the waterline’ (see also below § 5).88 Figure 3: Iceberg model – Monitoring prison violence On the tip of the iceberg there are events and outcomes (direct violence) such as violent acts between prisoners, assaults on prison staff, incidents of self-harm and suicide as well as other custodial deaths. High numbers of violent incidents can alert NPMs and signal the need for a more in-depth investigation.89 However, as violence often happens behind closed cell doors and remains unreported, such numbers should not be the only indicator for prison violence. Underneath the waterline there are patterns and underlying structures (structural violence), such as trends in how authority and leadership are exercised, regulations, procedures, programmes and policies as well as measures for their implementation and 88. A useful tool for monitoring in this regard is the: PRI and APT, Institutional Culture in Detention: A Framework for Preventive Monitoring (PRI and APT 2015) [PRI and APT, Institutional Culture]. 89. Such as the ad hoc visit of the Council of Europe’s CPT to the United Kingdom, which took place from 13 to 23 May 2019 due to persistently high levels of violence in the local male adult prisons and juvenile detention centres, as well as broader concerns regarding regimes, the use of force, segregation and use of means of restraint.: CPT,‘Report on the Visit to the United Kingdom’ (2020) CPT/Inf (2020) 18. Monitoring Prison Violence