Primary losses
Primary losses are the portion of a claim's cost, up to your state's split point, that counts at full weight in your EMR (experience modification rate) calculation. Every claim gets split into a primary portion and an excess portion, and the primary portion is what drives most of the difference between a mod at 1.0 and one above it. Because primary losses count fully regardless of claim size up to the split point, several smaller claims can push more total dollars into the fully weighted primary category than one large claim does, which is why claim frequency tends to hurt a mod more than severity. A safety program aimed at reducing how often claims happen, not just how severe the rare bad one is, generally has more effect on your mod for the same investment. Checking your loss runs for how many claims fall entirely within the primary range, rather than just looking at total dollars, gives a more accurate read on what's actually driving your rating.
See also
Source
Is your mod right?
Most contractors have never had the worksheet compared to the loss runs and payroll behind it. See what your mod is costing you, or get on the list for a full verification.