13.3 Premium Basis, Experience Modification, and Classification
Key Takeaways
- Workers comp premium is based on payroll: rate per $100 of payroll × (payroll ÷ 100), assigned by job classification code.
- The premium is estimated at inception and adjusted by a year-end audit because actual payroll rarely equals the estimate.
- The Experience Modification Factor (mod) compares an employer's actual losses to expected losses: above 1.00 surcharges premium, below 1.00 credits it.
- Classification codes group employees by the work they do; payroll is generally assigned to the governing class, with standard exceptions for clerical, outside sales, and drivers.
- Premium discounts and retrospective rating plans reward larger insureds for size and good loss experience.
Premium Is Built on Payroll
Workers comp is rated on payroll, not on values or limits. The base manual premium formula is:
Manual Premium = (Payroll ÷ 100) × Rate
The rate is a dollar figure per $100 of payroll that varies by classification code — a clerical office worker carries a low rate (low injury exposure) while a roofer carries a high one. Because the actual payroll for the year is unknown when the policy is written, the insurer charges an estimated premium at inception and performs a payroll audit at expiration to true it up. If actual payroll exceeded the estimate, the insured owes additional premium; if it was lower, a return premium is due.
Worked example: An employer expects $800,000 of payroll in a class rated at $3.50 per $100.
- Manual Premium = (800,000 ÷ 100) × 3.50 = 8,000 × 3.50 = $28,000.
If the audit shows actual payroll of $900,000, premium recomputes to (900,000 ÷ 100) × 3.50 = $31,500, and the insured owes an additional $3,500.
The Experience Modification Factor (the 'mod')
Larger, credible employers earn an experience modification factor that adjusts manual premium to reflect their own loss history. The mod compares the employer's actual losses to the expected losses for a business of its size and class:
Modified Premium = Manual Premium × Experience Mod
- A mod of 1.00 is exactly average.
- A mod above 1.00 (a debit mod) means worse-than-average losses — it surcharges premium.
- A mod below 1.00 (a credit mod) means better-than-average losses — it credits premium.
Worked example: Manual premium is $28,000 and the employer's mod is 0.85 (good experience):
- Modified Premium = 28,000 × 0.85 = $23,800 — a $4,200 saving.
If instead the mod were 1.25 (poor experience):
- Modified Premium = 28,000 × 1.25 = $35,000 — a $7,000 surcharge.
Exam Key: The mod is the single biggest lever an employer controls through safety. Below 1.00 = credit/discount; above 1.00 = debit/surcharge. It is applied to manual premium, before scheduled credits and premium discounts.
Classification Codes and Payroll Assignment
Every job is mapped to a classification code (NCCI or an independent state bureau) that carries its own rate. Most of an employer's payroll goes to the governing classification — the class describing the business's principal operation. Three groups are standard exceptions rated on their own lower codes regardless of the governing class:
- Clerical office employees
- Outside salespersons
- Drivers
| Element | Workers comp treatment |
|---|---|
| Premium base | Payroll (per $100) |
| Rate source | Classification code by job |
| Estimated vs. final | Estimated at inception; audited at expiration |
| Individual adjustment | Experience mod (actual vs. expected losses) |
| Size/experience reward | Premium discount; retrospective rating |
Large insureds may use a retrospective rating plan, where final premium is computed after the period from the insured's actual losses within a minimum/maximum band — strongly rewarding loss control.
Order of Operations and Premium Discounts
The pieces stack in a fixed sequence, and the exam may ask you to apply them in order:
- Manual premium = (payroll ÷ 100) × class rate, summed across all classifications.
- Experience modification = manual premium × mod (credit below 1.00, debit above).
- Schedule rating credits/debits for risk characteristics (where allowed by the state).
- Premium discount — a volume discount that grows with premium size, reflecting the fixed expenses spread over a larger account.
- Expense constant — a small flat charge added to every policy to cover issuing and auditing costs.
Worked example: An employer has $1,000,000 of payroll at a $4.00 rate and a 0.90 mod.
- Manual premium = (1,000,000 ÷ 100) × 4.00 = $40,000.
- Modified premium = 40,000 × 0.90 = $36,000.
A 5% premium discount would then reduce it to 36,000 × 0.95 = $34,200, before adding the expense constant.
The mod itself is computed from a multi-year window (typically three years, dropping the most recent year) so a single bad year does not swing it wildly, and it weights frequency over severity — many small claims push the mod up faster than one large claim, because frequent losses predict future losses better. That is why loss-control and return-to-work programs that cut the number of claims move the mod most.
Exam Key: The experience mod is applied to manual premium, then premium discounts apply. Do not apply the discount before the mod. The mod is the employer-specific lever; the discount is a size lever available to everyone of that premium size.
Premium Determination and the Experience Modifier
WC premium is built from payroll, classification, and experience, calculated as:
Premium = (Payroll / 100) x Classification Rate x Experience Modifier, then adjusted by schedule/dividend factors.
| Component | Role |
|---|---|
| Classification code | NCCI/state code grouping similar exposures (e.g., clerical vs. roofing); rate reflects hazard |
| Payroll (per $100) | The exposure base; estimated at inception, audited at year-end |
| Experience Modification Factor (mod) | Compares the insured's actual losses to expected losses for its class |
The experience mod is the key risk-management lever: a mod of 1.00 is average, below 1.00 (credit) rewards better-than-average loss experience and lowers premium, and above 1.00 (debit) raises it.
Exam trap: A mod below 1.00 is a credit (premium discount); a mod above 1.00 is a debit (surcharge) - safety performance directly drives cost. WC premium is estimated at inception and finalized by a payroll audit after the term, so the final premium can differ from the deposit. Retrospective rating plans adjust premium based on the insured's actual losses during the period, within a min/max, for large accounts.
An employer's manual premium is $40,000 and its experience modification factor is 1.20. What is the modified premium, and what does the factor indicate?
Workers compensation premium is primarily based on which exposure base?