The full NCCI/bureau formula involves state-specific tables and weighting factors that even most brokers don’t calculate by hand — that’s what the rating bureau’s software is for. But the logic behind it is straightforward once you see it laid out, and understanding it is what lets you actually manage your mod instead of just reacting to it at renewal.
Step 1: Expected Losses
The bureau starts by calculating what your losses “should” be, based on your payroll and class codes, using expected loss rates published for your trade and state. This is your baseline — the number your actual claims get measured against.
Step 2: Split Every Claim Into Primary and Excess
Each individual claim is divided at a state-specific dollar line called the split point. The portion of the claim below that line is “primary” and counts toward your mod almost dollar-for-dollar. The portion above the line is “excess” and is weighted far more lightly — often counted at only a fraction of its value.
Step 3: Why This Makes Frequency the Real Lever
Because primary losses are weighted so much more heavily than excess losses, claim frequency moves your mod more than the size of any single claim does. Three soft-tissue claims at $8,000 each will typically hurt your mod more than one unusual $150,000 claim, because the primary portion of each small claim counts almost in full, while most of that one large claim falls into the lightly-weighted excess bucket.
Step 4: The Ballast Value
A stabilizing factor — sometimes called a “ballast” — is built into the formula specifically so that one bad year for a smaller contractor doesn’t cause a wild, disproportionate swing in the mod. Larger payroll bases get more “credibility” in the formula, meaning their own claims history counts for more, for better or worse.
A Simplified Example
The real formula involves more variables than this, but the shape of the math looks roughly like this for an illustrative contractor:
| Component | Amount |
| Actual Primary Losses (claims below the split point) | $18,500 |
| Expected Excess Losses (held at the trade’s typical value) | $41,000 |
| Expected Losses (payroll × class code rate) | $52,000 |
| Illustrative Mod = (Primary + Expected Excess) ÷ Expected Losses | ≈ 1.14 |
This is a simplified illustration to show the shape of the math, not a substitute for an actual worksheet — real calculations use your specific state’s split point, D-ratios, and expected loss rates. For a precise number, request a worksheet review.
Want to see where your own numbers stand? Request a Scorecard Review and we’ll walk your actual worksheet through this same math.