Ignorant question here, but who grades SSEs is it SRG or the unit (ie ATC OPS/Training)?
SSEs are graded by the unit, however, they then go to a national moderation panel who recieve a briefing on the incident and will come up with an independent scoring. This is then compared with the unit score and after debate (and adjustment of the panel score if the debate dictates) then the moderation panels scoring will be the final SSE mark which NATS uses for management purposes.
Of course, marks may go up as well as down as a result of any moderation
