Our ideas about the purpose of prisons and the methods of dealing with prisoners have changed during the past fifty years. An increasing number of "repeaters" in our prisons strongly suggested that the policy of putting criminals into prison in order to get revenge against them for the crimes they committed was an economic waste and a serious danger to society. It meant that every year thousands of broken, hopeless, bitter men were turned out of prison unwilling and unable to earn an honest living.
Gradually, thoughtful people came to realize that the protection of society rather than revenge against the criminal should be the guide in the treatment of prisoners. Prison administrators began to take account of the fact that 97 per cent of those who go to prison come out sooner or later,  and that it is the job of an institution to try to send them out physically, mentally, and morally better than they were when they entered.
These ideas grew up during the period when our knowledge of human behavior was increasing. Research and study in the fields of medicine, psychology, the social sciences, education, and other subjects dealing with human beings added to our understanding of people and changed many of our older ideas about the reasons for their behavior. It was only natural, therefore, that specialists in these different fields of study should be called into the prison service to add their knowledge and skills to the practical experience and knowledge of those already in charge of penal institutions.
This fundamental change in our ideas about the purpose of imprisonment and the introduction of specialists into prison work had an immediate effect. It made the individual prisoner the important element in prison management. Mass treatment may provide sufficient punishment, but experience has demonstrated that it is an ineffectual method of bringing about rehabilitation and consequently ineffectual as a program of public protection.
One of the first steps to be taken in establishing a program of individualized treatment is to put offenders presenting more or less similar problems into the same prison, and provide for different types of prisons. In fact, the legislation which authorized the organization of the Bureau of Prisons specifies that the institutions shall be so planned and limited in size as to facilitate the development of an integrated federal penal and correction system. Such a system must assure the proper classification and segregation of federal prisoners according to their character, the crime committed, and other factors which are considered in providing an individualized system of discipline, care, and treatment.
All kinds of criminal offenders are committed to federal  prisons -- young and old, inexperienced and hardened, healthy and diseased. Some are intelligent and in good mental health, others are mentally defective and mentally ill. Still others are addicted to narcotics. From the standpoint of custody risks, prisoners range from those who can be trusted in an open camp to those who present a serious escape hazard to the most secure prison man can devise. It is obviously not wise to place offenders with such different characteristics in the same institution and to expect the same degree of individualized treatment and the same constructive results that could be obtained if each prison housed persons of the same general kind.
Consequently, reformatories such as the ones at Chillicothe and El Reno have been established for youthful offenders who do not have a record of serious or extensive criminal experience. The penitentiaries are of different types. Some have been designated for the confinement of the better rehabilitative prospects among offenders above the reformatory age, others for habitual but tractable offenders. At the other extreme, Alcatraz has been designated for offenders who are serious escape risks and serious conduct problems. A number of correctional institutions have been provided to care for the short-sentence offenders and for those who do not require the security facilities of a penitentiary. Special institutions have been established for the treatment of narcotic addicts and for chronic medical and mental cases.
Classification should not be thought of meaning only the proper segregation of offenders in different institutions. Broadly, it means the systematic study and treatment of individual prisoners. It implies that each case is carefully and intensively analyzed to determine, if possible, what factors have contributed to the prisoner's violation of the law, and it implies the development of a program of treatment and training which will enable him to live a law-abiding life after his release from prison.
Factors which the classification board considers are: (1) the necessary degree of custody and supervision; (2) whether the prisoner should be transferred to another institution more suitable to his needs; (3) the amount of social service which  his family may need; (4) the necessary medical and neuropsychiatric treatment; (5) the vocational training which he should receive; (7) the degree of religious training recommended.
[GOGA-JPS:HDC 419:pp. 93-96]