Psychiatric hospitals, also known as mental health hospitals, and mental health units', are hospitals or wards specializing in the treatment of serious mental disorders, such as major depressive disorder, schizophrenia and bipolar disorder. Psychiatric hospitals vary widely in their size and grading. Some hospitals may specialize only in short term or outpatient therapy for low-risk patients. Others may specialize in the temporary or permanent care of residents who, as a result of a psychological disorder, require routine assistance, treatment, or a specialized and controlled environment. Patients are often admitted on a voluntary basis, but people whom psychiatrists believe may pose a significant danger to themselves or others may be subject to involuntary commitment. Psychiatric hospitals may also be referred to as psychiatric wards or units (or "psych" wards/units) when they are a subunit of a regular hospital.