Safety Event Groups seems to be a huge part of CSA that appears simple, but really just add to the confusion.  In August, 2010, the FMCSA shifted the methodology from Peer Groups to Safety Event Group when they stopped using Average Power Units (APU) to group fleets.  This lead to a lot of concern as suddenly many fleets saw drastic shifts in their percentile rank.  Why the sudden shift in rank if there wasn’t a sudden shift in performance?

Let’s start here: What is a Safety Event Group?
Fleets are grouped, in each BASIC, by the number of safety events (relevant inspections, crashes, or inspections with violations).  These are the groups fleets are ranked against to give the percentile rank.  “Safety event groups do not compare carriers by the commodities they haul or their industry segment.”
From the FMCSA’s CSA FAQ page:

So, fleets are compared to carriers who have a similar number of “safety events.”  Why is that important to me?  For many fleets, this is a concern because they are being compared to fleets who have completely different lines of business.  For example, a fleet of 100 bread trucks making deliveries compared to a fleet of 10 flatbed trucks may look the same to the FMCSA.  Even if this isn’t a concern, it’s still important to know what Safety Event you belong to in case you change Safety Event Groups.

Yes.  Change Safety Event Groups.

This movement of carriers between different Safety Event Groups is also why it’s hard to have a good list of which fleets are in which Safety Event Group.  And what happens when you change Safety Event Groups?  Well, you’re now being compared to a completely new set of carriers.  So that 10 flatbed trucks may now be competing with a fleet of 100 intrastate refer units.  Even if their internal performance stays the same, if their comparison groups shifts, so can their Percentile Rank.

Check out this fleet.  Their internal performance stayed virtually the same (BASIC Measure from 0.9 to a 0.91; total inspections with violations from 68 to 69; and total violations no change at 85).  But the fleet shifted from Safety Event Group 4 to Safety Event Group 5 in Fatigued Driving and jumped more than 15 percentage points.

And suddenly they’re in “symbol for alert” status: above the threshold, and being scrutinized even closer.