You can map year on year employment groups in 'Back Office -> Survey Processes -> Year On Year Employment Group Mappings', but one thing you should be aware of when doing this is that you must complete this in order of oldest entry first up to the newest one.
For example, you may need to map employment groups for Accreditation from the current entry (2015) back to a companies first Accreditation entry (2011). Since the newest entries appear first in the drop down lists, it is all too easy to map these first. This can cause issues in WIT, where you will not be able to see year on year comparisons between say 2015 and 2013. Instead you might only see a comparison between 2015 & 2014.
For this reason, you will need to map the groups in the following order:
2011 - 2012
2012 - 2013
2013 - 2014
2014 - 2015