This figure is taken from a snapshot of data. ISPs are prioritised for transfer to open conditions using the date the Secretary of State approved their move. Whether prisoners are serving an IPP or a life sentence is not relevant to the prioritisation criteria and as a result was not, at that time, recorded as part of the process.
Providing a figure for IPPs alone would incur disproportionate costs as that data has not been retained in a recorded form. In order to establish the number of prisoners serving IPP sentences at the time the snapshot was taken, retrospective checks of individual prisoners' records would have to be performed to recreate the list at the time it was generated.
This data is subject to variation due to progress of individual cases including prisoners removal from open conditions following adverse developments and Parole Board decisions following review.
This figure was drawn from administrative IT systems which, as with any large scale recording system, are subject to possible errors with data entry and processing.