13.3 Premium Basis, Experience Modification, and Classification

How Workers Compensation Premium Is Built

Workers compensation premium is payroll-based and audited, which is unusual among property and casualty lines (most lines rate on values, units, or receipts). The exam consistently tests four things: the manual-premium formula, the classification system, the experience modification factor, and the premium audit. Master the order of operations and you can answer almost any WC rating question.

The Premium Formula

The manual premium for each class of work is:

Manual Premium = (Payroll / 100) x Rate per $100 of payroll

Payroll is divided by 100 because WC rates are quoted per $100 of remuneration. Each job is assigned a classification code (an NCCI four-digit class code, for example 8810 clerical office, 8742 outside sales, 5403 carpentry, 7380 drivers) and each class carries its own manual rate reflecting the hazard of that work. A roofer's rate may be many dollars per $100 while a clerk's rate is a few cents, because the loss potential differs enormously. The governing classification is the highest-rated class that describes the principal business; standard exceptions like clerical and outside sales are split out separately.

Worked Numeric: Manual Premium

A contractor has $600,000 of carpentry payroll (class 5403) at a rate of $9.50 per $100:

• $600,000 / 100 = 6,000 payroll units.
• 6,000 x $9.50 = $57,000 manual premium for the carpentry class.

The same contractor also has $200,000 of clerical payroll (class 8810) at $0.40 per $100:

• $200,000 / 100 = 2,000 units; 2,000 x $0.40 = $800.

Total manual premium across both classes = $57,000 + $800 = $57,800. Notice how a small clerical rate produces a tiny premium even on substantial payroll, which is why splitting payroll into the correct classes matters so much.

The Experience Modification Factor (the Mod)

Employers large enough to be statistically credible qualify for experience rating. The experience modification factor ("the mod") compares the employer's actual losses to the expected losses for businesses of the same class and size:

• Mod = 1.00 is exactly average (neutral; no change to premium).
• Mod below 1.00 is a credit mod: better-than-average loss experience, so premium decreases.
• Mod above 1.00 is a debit mod: worse-than-average loss experience, so premium increases.

The mod rewards safe employers and penalizes unsafe ones, and it is the single largest cost lever an insured controls through loss prevention, return-to-work programs, and claims management. Frequency of small claims usually drives the mod up more than one large claim, because rating formulas weight expected frequency heavily.

Worked Numeric: Applying the Mod

Use the $57,800 manual premium above with a 0.85 experience mod:

• $57,800 x 0.85 = $49,130 modified premium (the credit mod saves the employer $8,670).

Now apply a 1.20 debit mod to the same manual premium:

• $57,800 x 1.20 = $69,360 modified premium (a surcharge of $11,560 versus manual).

The spread between the credit and debit outcomes here is over $20,000 on identical payroll, which dramatizes why employers invest in safety.

Schedule Rating and Premium Discount

After the mod, an underwriter may apply schedule rating credits or debits for risk-specific characteristics not captured in the class rate (premises condition, management cooperation, employee training). Large policies then receive a premium discount, a graduated volume discount recognizing that fixed expenses spread over a larger premium. The order of operations the exam expects is:

1. Compute manual premium (payroll / 100 x rate) for each class and total them.
2. Multiply by the experience mod to get the modified premium.
3. Apply schedule rating credits/debits.
4. Apply the premium discount and add the expense constant to reach the final premium.

The Premium Audit

WC premium charged at policy inception is only an estimate, based on the insured's estimated payroll for the coming year. Because actual payroll is rarely known in advance, the insurer performs a premium audit after the policy expires, examining payroll records, tax filings, and contractor payments:

• If actual payroll exceeded the estimate, the insured owes additional premium.
• If actual payroll was lower than the estimate, the insured receives a return premium.

The insured's duty to keep records and permit the audit is in Part Five and Part Four of the policy. Refusing the audit can let the insurer apply an estimated-payroll surcharge.

Remuneration counted as payroll includes wages, salaries, commissions, bonuses, holiday and vacation pay, and the value of certain in-kind compensation such as lodging. It excludes tips, severance pay, and (in most states) the overtime premium portion, meaning the extra one-half of time-and-a-half is removed so overtime is counted only at the straight-time rate.

Common Trap: Owner and Officer Payroll

Sole proprietors and partners are usually excluded from coverage (and from payroll) unless they affirmatively elect to be covered. Executive officers of a corporation are generally included, but their payroll is capped between a state minimum and maximum weekly amount rather than counted at full salary, so a highly paid officer does not generate runaway premium. This min/max officer payroll rule is a recurring exam detail.

Loss-Cost Multipliers and the Combination of Rating Factors

Most states publish advisory loss costs (the pure-loss portion of the rate) through NCCI; each insurer then applies its own loss-cost multiplier (LCM) to load for expenses and profit, producing the filed manual rate. The full rating chain is: payroll/100 x rate, then x experience mod, then schedule-rating credits/debits, then the premium discount, plus the expense constant and any terrorism/catastrophe charges.

Trap: The experience mod is applied to the manual premium, not to the final premium after discounts - apply factors in the correct order or the dollar answer is wrong. And officer payroll is counted only between the state minimum and maximum, never at full salary.