Prison Service Standards are instructions from Prison Service HQ which set out the national requirements for a prison. The Prison Service Standards Audit Unit visits each prison every two years to measure its compliance with these standards. It does not monitor every area but focuses on the standards most closely associated with the Prison Service's strategic priorities (such as drug strategy and education). The Unit allocates an overall rating for the prison of either: superior, good, acceptable, deficient or unacceptable. We would have given any prison that received a deficient or unacceptable rating a red indicator in our traffic light matrix, although as it turned out, the prisons in our study were rated acceptable or above.