The traffic light matrix combines qualitative and quantitative data in an intuitively sensible way in order to make overall performance comparisons between the prisons in our study. These prisons were not randomly selected from the prison estate but were chosen for specific reasons of comparability i.e. those public sector prisons which were deemed the best comparator prisons to the PFI prisons. A prison that is awarded an overall red indicator in our matrix should be seen in the context of underperforming in relation to the other prisons in our study and not necessarily underperforming in the prison estate as a whole.
The quantitative data used in this methodology was from the 2001/02 period, whereas the qualitative data ranged from September 2000 to May 2002. It was necessary to widen the time period for the qualitative data in order to incorporate a sensible number of prison inspection reports. Too short a period and the number of reports available would be small, too long a period and the comments within them run the risk of being out of date.