A few things are used for the metrics, Time & attendance, Quality notifications, Open disciplinary's, Skill sets, Core behaviours, PDR (personal development reviews) scores for the last 12 months.
Don't get me wrong a lot of the people picked to go have made their beds over the last 12 months and now they have to live with their actions but there are some that will be leaving that have not done a lot wrong but the cuts needed are very deep (half the workforce) and it has encroached on some of the better people.