Milieu therapy is a form of psychotherapy that involves the use of therapeutic communities. Patients join a group of around 30, for between 9 and 18 months. During their stay, patients are encouraged to take responsibility for themselves and the others within the unit, based upon a hierarchy of collective punishments. If one patient violates the rules the entire ward is punished to varying extents based upon their cooperation with the staff. Milieu therapy is thought to be of value in treating personality disorders and behavioural problems. Henderson Hospital, in London, used this type of psychotherapy, as does Forest Heights Lodge in Evergreen, CO.