The idea
Two businesses with the same payroll and class codes can have different injury records. The experience mod lets the premium reflect that record. Expected losses are calculated from your payroll by class; actual losses from your claims; the ratio, with weighting that limits the effect of one large claim, becomes the factor.
What moves it
Frequency more than severity. Several small claims move the mod more than one large one, because the formula weights the first dollars of each claim most heavily. Prompt reporting, return-to-work programs and medical-only claims (which many states discount) keep it down.
Where to find yours
On the experience rating worksheet from your state's rating bureau or the national bureau, usually sent to your agent each year before renewal. Upload it to the wallet and the renewal application carries it to the carriers, and your general contractor's certificate request can show it.