The rate of imprisonment is 70 per 100,000 population. The prison population rose sharply in the years following 1975, as a result of greater numbers of arrests of drug abusers and drug traffickers, intensified enforcement efforts directed at criminals and Secret Societies, and the courts' introduction of severer penalties. It is common for two or three prisoners to be housed in a cell designed for one. Prisoners are classified according to age, character, and previous history. The high rate of illiteracy among inmates has made education an integral part of the rehabilitation program. The majority of offenders also have no vocational skills and therefore receive vocational training. The use of borstal farms and agricultural training are major recent trends. Malaysian penal institutions have not introduced work release and furlough. Explicit rules govern security measures, inspections and searches of prison officers, disciplinary measures, and the use of firearms and restraining devices. Prisoners have the right to visits, correspondence with censorship, religious worship, access to libraries and entertainment, and personal freedom and privacy. The prison's medical officer examines each prisoner after admission. Nearly all of the prison rules on the treatment of inmates conform with those in the United Nations Minimum Standards Rules. The Prison Officers' Training School provides inservice training courses for prison staff, who are selected on the basis of integrity, humanitarian approach, competence, physical fitness, and completion of secondary school.