There are a number of criteria for discretionary release decision-making. The ultimate one is public safety risk, which is assessed by an independent group: the National Parole Board. Part of that risk appraisal incorporates criminal history background, as well as what they did while they were in prison, whether or not they behaved, they got involved in security-related incidents or caused more disturbances--
On April 29th, 2010. See this statement in context.