The mental model

A premium is the carrier's price for accepting a defined risk. The premium has to cover expected losses, expenses, profit, and a return on the capital required to back the policy. Rating is the structured methodology for getting from risk characteristics to a defensible premium number.

For admitted lines, rating happens through filed rates: a published rate manual that defines the base rate per unit of exposure, the modifications that apply, and the construction of the final premium. The rate filing is reviewed and approved by state insurance departments before it can be used. For surplus lines, rating is more flexible; carriers price based on internal pricing models and underwriter judgment without state-by-state filing approval. Both regimes share the same conceptual machinery: identify the exposure base, apply a base rate, modify for individual risk characteristics, calculate the premium. The vocabulary is dense and the mechanics differ across lines, but the pattern is consistent. Knowing where each modification comes from and what it represents is essential for any underwriter conversation about pricing or for any pricing-related document automation work.

Exposure base

The exposure base is the unit of measurement that scales with the underlying risk. Larger exposure means more expected losses (all else equal). Each line of business has a characteristic exposure base.

Why exposure base matters

The exposure base must scale with expected losses for the rating to work. A line that is rated per vehicle but where loss potential scales with miles driven will mis-price fleets that operate intensively (long-haul trucking) compared to fleets that operate lightly (local delivery). Carriers periodically revisit exposure bases when the relationship between the base and expected losses degrades.

Premium audit

For lines with variable exposure (WC, GL on payroll or sales), the policy is initially priced on estimated exposure and audited at expiration. The audit reconciles actual exposure (actual payroll, actual sales) against estimate and generates a return premium or additional premium. Premium audit is a substantial operational function in itself.

Manual rate

The manual rate is the published rate per unit of exposure. For admitted lines, it comes from the carrier's filed rate manual.

How the manual rate is built

• Loss costs. The actuarial estimate of expected losses per unit of exposure for the class. Pure loss cost reflects only expected losses, with no expense or profit load.
• Loss adjustment expense provision. Add-on for ALAE and ULAE.
• General expense provision. Add-on for carrier overhead, commissions, premium taxes.
• Profit and contingency provision. Add-on for target underwriting profit and adverse-deviation buffer.
• Investment income offset. Reduction reflecting expected investment earnings on premium and reserves.

The bureau loss costs

For most U.S. commercial lines, ISO and the National Council on Compensation Insurance (NCCI) develop and publish loss costs. Carriers either adopt the bureau loss costs (with an LCM, see below) or develop their own. The NCCI loss costs for workers compensation are the dominant rating reference for that line in most states.

Class plans

Class codes group similar risks for rating purposes. WC has the NCCI class code system (700+ codes covering specific occupations). GL has the ISO general liability class plan. Property has the ISO commercial lines manual class plan. Each class has its own filed rate.

Territory and state variation

Rates vary by state and often by territory within a state (urban vs rural, coastal vs inland). The same class code can have very different rates in different jurisdictions reflecting venue differences in litigation, claim frequency, and severity.

Loss cost multiplier

Carriers that adopt bureau loss costs apply a Loss Cost Multiplier (LCM) to convert pure loss cost into a usable rate.

Mechanics

LCM = (1 + expense provision + profit provision) / (1 - taxes and other variable expenses). The LCM scales the bureau loss cost up to include all of the loadings the carrier needs in its rate.

Variation across carriers

Different carriers apply different LCMs reflecting their expense ratios, target profit, and competitive positioning. A carrier with a low expense ratio can apply a lower LCM and offer competitive rates while still hitting profit targets. The LCM is a public filed number; brokers and competitors observe it.

The competitive math

• Bureau pure loss cost: 1.50 per $100 of exposure.
• Carrier A LCM: 1.45 → filed rate 2.18.
• Carrier B LCM: 1.55 → filed rate 2.33.
• Carrier C LCM: 1.35 → filed rate 2.03.

The same risk would be quoted at different premiums by these three carriers based purely on the LCM differences. Other modifications (experience mod, schedule mod) further differentiate.

Experience modification

The experience mod adjusts the manual rate for the specific insured's actual loss history. It is the most consequential mod in workers compensation and applies (in different forms) in several other lines.

The mechanics

The mod compares the insured's actual losses to expected losses for the same exposure. A mod of 1.00 means actual losses match expected. A mod of 0.85 means actual losses are 15% below expected; the insured's premium is reduced 15% accordingly. A mod of 1.15 means actual losses are 15% above expected; the insured's premium is increased 15%.

The WC experience mod

The NCCI Experience Rating Plan governs WC experience modification in most states. The mod is calculated annually using a rolling three-year window of the insured's losses, excluding the most recent policy year (which is still developing). The calculation uses split rating: primary losses (small and medium claims) are weighted differently than excess losses (severe claims). This dampens the volatility from individual large claims.

The published mod

The NCCI calculates and publishes the WC experience mod for each eligible employer. The mod is on the policy and visible to the broker, the insured, and the carrier. Insureds with high mods face premium surcharges; insureds with low mods enjoy discounts. The mod is a strong incentive for active workplace safety management.

Eligibility

Experience modification requires sufficient premium to be statistically credible. Small employers with insufficient premium are not experience-rated; their rate is the manual rate. The eligibility threshold varies by state but typically requires premium in the range of $5,000-$10,000 per year for at least three years.

Loss-rated and experience-rated GL

For larger insureds, GL pricing increasingly uses loss-rating or experience-rating analogous to the WC mod. The mechanics differ from WC's NCCI-based mod, but the principle is the same: actual loss experience modifies the premium up or down from the manual rate.

Schedule modification and IRPM

Beyond experience-driven mods, carriers apply judgment-driven mods that reflect risk characteristics not fully captured in the loss history.

Schedule rating

The carrier's filed schedule mod factors. Specific risk attributes (management experience, employee training, premise safety, claim handling, internal controls) generate plus or minus modifications within filed bounds (often ±25% in WC, ±40% in GL). The underwriter applies the schedule mod based on the specific risk's characteristics.

IRPM (Individual Risk Premium Modification)

Filed by some carriers as a separate mod or as the schedule mod itself. IRPM allows underwriter judgment to flex the rate within filed bounds based on factors not captured in the experience mod. Common IRPM categories: management, location, premises and operations, equipment maintenance, employee training and safety, classification of risk, claim handling.

The discretion question

Schedule mods and IRPM give underwriters meaningful pricing discretion. Two underwriters looking at the same risk can produce different prices through schedule mod variation. Carriers monitor schedule mod usage to ensure consistency and compliance with filed parameters.

Tier rating

Some carriers use tier rating: the carrier maintains multiple "tiers" of business, each with different rate levels, and assigns risks to tiers based on overall risk profile. The tier assignment may be model-driven, judgment-driven, or hybrid. Tier rating is most common in personal lines but appears in some commercial program business.

Credibility theory

The actuarial machinery underlying experience modification is credibility theory. The principle is straightforward but the implementation requires care.

The credibility question

How much weight should the insured's own loss experience receive vs the class average? An insured with $5M in payroll has limited experience to draw conclusions from. An insured with $50M in payroll has more, but still not unlimited. Credibility quantifies the answer.

The credibility formula

Credibility is expressed as Z, a value between 0 and 1. The credibility-adjusted estimate is Z × (insured's actual experience) + (1 - Z) × (class average). When Z = 0, the rate is the class manual; when Z = 1, the rate is fully driven by the insured's experience.

How credibility scales

Credibility increases with exposure size. Small employers have low credibility; large employers approach full credibility. The mathematics involves the variance of expected claim counts and severity at different exposure levels.

The square-root rule

A useful approximation: credibility scales with the square root of exposure. Doubling exposure does not double credibility; it increases credibility by approximately √2 (about 41%). To go from 50% credibility to 100% credibility requires roughly four times the exposure size.

Why this matters in practice

The experience mod calculation embeds credibility weighting. Smaller insureds have their losses dampened toward the class average; larger insureds have their losses reflected more fully. This is why a single bad year affects a large WC insured more than a small WC insured: the credibility weight is higher.

Rate filings and the regulatory layer

For admitted lines, rating happens through filed rates that have been reviewed by state insurance regulators.

The filing process

A carrier proposes a rate change (new rates, modifications to existing rates) and files it with the state insurance department. The state reviews the filing for actuarial soundness, statutory compliance, and (in some states) prior approval before the rate becomes effective.

Filing regimes

• Prior approval. The rate cannot be used until the state explicitly approves it. Common in highly regulated lines and certain states.
• File-and-use. The carrier files and can begin using the rate after a defined waiting period unless the state objects.
• Use-and-file. The carrier begins using the rate and files within a defined window. Less common.
• No file. Some lines (large commercial, surplus lines) are exempt from rate filing.

Filing components

• The proposed rate change (overall and by territory, class, etc.).
• Actuarial support: loss experience analysis, trend analysis, expense analysis, profit provision.
• Form changes: any changes to policy forms accompanying the rate change.
• Class plan changes: any changes to class definitions or rating territories.
• Schedule mod and LCM filings.

Rate adequacy and rate inadequacy

States review filings to ensure rates are not excessive (gouging) and not inadequate (underpricing leading to insolvency). The inadequacy concern has driven recent state actions in property markets where carriers want to charge more for catastrophe-exposed coverage and states have resisted. The result has been carrier withdrawal from the highest-risk markets.

Where IDP earns its keep

Rating itself is a structured calculation that lives in rating engines and pricing models, not directly in IDP. But the inputs to rating come from the submission documents that IDP processes. Exposure data extraction, class code assignment, schedule mod factor lookup, prior carrier rate comparison, premium audit reconciliation. The closer the input data is to the rating engine, the higher the leverage of accurate extraction.