At some facilities, processing early releases takes too long, while others fail to sufficiently monitor inmates. At other prisons, inmates are not even eligible for early release - because staff has fallen behind on paperwork.
The variety of procedures leads in inequality among convicts, the institute declares.
During the early parole system, which was adopted in 2006, prisoners can live at home for six months before their parole officially begins.
The study was based on data from 2008 and earlier.
The institute was established by the Ministry of Justice but is mandated to carry out impartial research on legal policy.
