The number of inmates held in solitary confinement in the United States has been notoriously difficult to determine. Most states do not publish the relevant data, and many do not even collect it. Attempts to come up with a figure have been denounced as imperfect, based on state-by-state variances and shortcomings in data-gathering and in conceptions of what constitutes solitary confinement.

A widely accepted 2005 study found that some 25,000 prisoners were being held in supermax prisons around the country. And in the last year, that figure seems to dominate in the mainstream press. The Washington Post, in a recent front-page article on solitary confinement in Virginia, noted that ’44 states…use solitary confinement,” and cited an ‘estimated 25,000 people in solitary in the nation’s state and federal prisons.” The problem here is that the 25,000 figure (as well as the 44) applies to supermax prisons only. It does not claim to account for the tens of thousands of additional prisoners held in the Secure Housing Units, Restricted Housing Units, Special Management Units and other isolation cells in prisons and jails around the country. Yet it is being cited as a total for the nation’s overall use of solitary confinement.

An alternative figure […]