13.3 Premium Basis, Experience Modification, and Classification

Premium Is Not Fixed at Issue

Unlike many property policies with a flat annual premium, workers' compensation premium is a moving target. The insurer estimates payroll and class rates at inception, applies an experience modification factor, and then audits actual payroll after the policy term. The national P&C exam expects you to know the formula steps and to run the arithmetic.

Step 1: Classification Codes

Every job type is assigned a National Council on Compensation Insurance (NCCI) classification code with its own rate per $100 of payroll. Rates reflect the historical injury frequency and severity of that occupation—a clerical code may cost cents per $100; roofing may cost many dollars.

When an employer has multiple classes, the governing classification is generally the standard (non-clerical, non-standard-exception) class with the largest payroll. Clerical employees, outside salespersons, and drivers are often standard exceptions rated in their own lower-cost classes.

Misclassification—placing high-hazard work in a low-rate code—is fraud and triggers additional premium, penalties, and possible cancellation.

Step 2: Manual (Exposure) Premium

The exposure base is payroll, and the rate is quoted per $100:

Manual Premium = (Payroll ÷ 100) × Rate

Worked example — single class: $60,000 payroll in a class rated at $5.00 per $100:

• $60,000 ÷ 100 = 600 units
• 600 × $5.00 = $3,000 manual premium

Worked example — multiple classes: $60,000 manufacturing at $5.00 plus $150,000 clerical at $0.20:

• Manufacturing: 600 × $5.00 = $3,000
• Clerical: 1,500 × $0.20 = $300
• Combined manual premium = $3,300

Step 3: Experience Modification (the Mod)

The experience modification factor compares the employer's actual losses to expected losses for its size and class mix over an experience period (typically three years, excluding the most recent policy year).

Worked example: Manual premium $3,300, experience mod 1.20:

• $3,300 × 1.20 = $3,960 modified premium (20% surcharge)

A mod of 0.85 on the same manual premium yields $3,300 × 0.85 = $2,805 (15% credit).

Why Frequency Beats Severity

The mod formula splits each claim into a primary portion (first dollars, up to a split point) and an excess portion. Primary losses count at full weight; excess losses are heavily discounted. Result: many small claims drive the mod up faster than one large claim of equal total dollars—because each claim contributes its own primary amount. The system deliberately penalizes frequency to incentivize loss prevention.

Exam trap: "One $50,000 claim hurts the mod more than five $10,000 claims." That is usually false—five claims each add a full primary loss.

Eligibility and Portability

Small accounts below the credibility threshold are not experience-rated and pay manual premium at a 1.00 mod. Once eligible, the mod is promulgated by NCCI or the state rating bureau—not chosen by the insurer—and follows the employer across carriers. Switching insurers does not escape a poor mod.

Merit rating plans grant smaller, not-yet-experience-rated employers a flat credit or debit based on claim count. Retrospective rating adjusts a large insured's final premium after the term based on actual losses within contractual maximum and minimum bounds.

Step 4: Schedule and Other Modifications

After the mod, insurers may apply:

• Schedule credits or debits for risk characteristics (safety programs, management, premises)
• Premium discount for size (volume discounts on large premiums)
• Expense constant and minimum premium on small accounts

Step 5: The Premium Audit

Policies are issued on estimated payroll. At year-end the insurer conducts a premium audit comparing estimated to actual payroll and adjusts premium accordingly.

The auditor reviews payroll records, tax filings, and certificates of insurance for subcontractors. Missing certificates for uninsured subs are a frequent source of surprise additional premium.

Putting It All Together

Assigned Risk (Residual Market)

An employer that cannot obtain coverage in the voluntary market—because of a high mod, severe loss history, or hazardous class—is placed in the assigned risk plan (residual market). Coverage is guaranteed but priced higher, and the experience mod still applies. NCCI administers assigned risk in most states, ensuring statutorily required Part One coverage is always obtainable—similar to assigned-risk auto plans.

Workers' compensation is often described as a line that prices safety: poor loss experience raises the mod, which raises premium, which can eventually push the employer into assigned risk. Strong safety programs and prompt return-to-work efforts are not just operational best practices—they are premium management.