I think Mr. Head would be in a better position to answer what percentage of people who are in there have a drug crime or drug offence as their principal offence. It will vary. As you pointed out, one quarter of all the prisoners in the United States--which incidentally incarcerates one quarter of all the human beings who are imprisoned on earth--are in prison for drug-related crimes, most of them non-violent. But again, Mr. Head might be in a better position to answer that question than I am with regard to the specific recent statistics.
On October 18th, 2011. See this statement in context.