Category C prisoners are not held in category D prisons. All prisoners are assessed as to their risk of escape or abscond, which ensures allocation of prisoners to a prison providing appropriate levels of security. Only those considered suitable would be held in category D open conditions.
The information required for such an assessment is not collated centrally and could be obtained only by asking every prison to review the categorisation history of a sufficiently large sample group of prisoners. This could not be done without incurring disproportionate cost.