Experience rating and the mod: what California, Washington and British Columbia say about workers' compensation

This page reports what the official source says. It is not legal advice: ask the office named here. It covers three places, one source each way of describing the same idea, and says nothing about any tree firm's own number. Where a figure appears, the page names the rule and the date it took effect. All pages were read on October 9, 2026.

California: the experience modification

The California Department of Insurance says the base premium comes from a classification code and its rate, applied per USD 100 of payroll, and is then modified by rating plans and by an experience modification. The modification is calculated from payroll and loss information that insurers must send each year to the Workers' Compensation Insurance Rating Bureau (WCIRB). The bureau uses a mathematical formula approved by the department, which takes into account the employer's payroll and losses, both paid losses and loss reserves, for an experience period.

The department says the modification compares the employer's loss history with all other employers in the same industry that are similar in size. Generally, it says, a modification below 100 percent reflects better-than-average experience and one above 100 percent reflects worse-than-average experience. The same guide says the bureau can issue modifications from reported losses without unaudited payroll, which it says typically produces a higher modification than in earlier years. See workers' compensation premium audits for the audit side.

Washington: the rule in the administrative code

In Washington, the rules sit in chapter 296-17 of the Washington Administrative Code. WAC 296-17-855, filed November 26, 2025 and effective January 1, 2026, says the basis is a comparison of the actual losses charged to an employer during the experience period with the expected losses for an average employer reporting the same exposures in each classification, weighted for how credible the employer's own experience is, given its volume. The rule says the expected losses are summed over the three years of the experience period, using the classification expected loss rates in its tables.

Point in WAC 296-17-855 and 296-17-870	What the rule says
Factor	The credible actual primary loss plus the credible actual excess loss, divided by the expected loss
Primary loss	For a claim of less than USD 28,180 the full value of the claim is primary loss; above that, a formula in the rule splits the claim into primary and excess parts
Claims without disability benefits	Actual losses are first reduced by the lesser of USD 4,100 or the total cost of the claim. The rule says USD 4,100 is twice the average case incurred cost of these claims during the three-year period
Which claims count	Claims with a date of injury during the experience period, evaluated on the valuation date, June 1, seven months before the effective date of premium rates
Changes after the valuation date	No claim value is revised between valuation dates, with listed exceptions such as a mistake other than error of judgment, a third party recovery, a second injury claim, or a claim later found noncompensable. No retroactive adjustment is made for rating periods more than ten years before the status change

British Columbia: the experience rating adjustment

WorkSafeBC states its premium formula as the industry's base premium rate, plus or minus the firm's experience rating, times the firm's assessable payroll. It says a firm with claim costs lower than the average for similar sized businesses in its rate group can earn discounts of up to 50 percent over time, and one with higher than average claim costs may face surcharges of up to 100 percent over time. The discount or surcharge is called the experience rating adjustment, and it is applied to the base rate to give the net premium rate.

The page gives a worked example: a base rate of 3.00 percent with a 10 percent discount gives a net rate of 2.70 percent, and on an assessable payroll of CAD 100,000 the premium is CAD 2,700, a difference of CAD 300. WorkSafeBC's premium rate letter, it says, confirms the classification, gives the base premium rate, shows any experience rating adjustments, and gives the net premium rate and the maximum assessable earnings per worker.

Side by side

Place	Name	How it is stated
California	Experience modification	A comparison to similar sized employers in the same industry; below 100 percent is better than average
Washington	Experience modification factor	Credible actual losses over expected losses for the same classifications
British Columbia	Experience rating adjustment	A discount of up to 50 percent or a surcharge of up to 100 percent on the base rate over time

None of the pages says what any tree service's modification or rate is. For the published class codes and rates, see workers' comp for tree work in the US.