The Factor That Prices Your Safety Record Into Every Premium
Workers compensation premiums are adjusted by a factor comparing an employer claims history against the average for its industry. The calculation weights claim frequency more heavily than severity, which is deliberate.
Pricing an Employer Against Its Own Record
Workers compensation insurance covers medical costs and lost wages for employees injured at work. Base premiums are calculated from payroll multiplied by a rate specific to each job classification, since roofers and office administrators present very different risks.
That produces a price reflecting what the employer does. It says nothing about how safely the employer does it.
The experience modification factor, universally called the mod, adjusts for that. It compares an employer actual claims experience against the expected experience for a business of that size in those classifications, and multiplies the premium accordingly.
| Mod | Meaning | Premium Effect |
|---|---|---|
| 1.00 | Exactly average for the class | No adjustment |
| 0.80 | Better than average | 20 percent discount |
| 1.35 | Worse than average | 35 percent surcharge |
Frequency Counts More Than Severity
The feature that surprises employers is how the calculation weights claims.
The formula separates each claim into a primary portion, up to a split point, and an excess portion above it. The primary portion enters the calculation at full weight. The excess portion is heavily discounted.
The consequence is that five small claims affect the mod far more than one large claim of the same total value.
This is deliberate and it is defensible. Claim frequency is a reasonably reliable indicator of how safely a workplace operates and is largely within the employer control. Claim severity contains a large element of chance, since the same fall can produce a bruise or a permanent injury depending on how somebody lands.
Rating an employer on frequency therefore measures something it can influence, and rating on severity would substantially measure luck.
The formula is designed to price safety management rather than outcomes. That is why a series of minor recordable injuries costs an employer more than a single catastrophic one, which strikes people as backwards until the reasoning is stated.
The Timing
The calculation uses a defined experience period, typically three years, ending about a year before the policy period. The most recent year is excluded to allow claims to develop.
That lag has two effects. A safety improvement takes years to show in the mod, which is discouraging for an employer trying to fix a problem. And a bad year continues affecting premiums for three subsequent years after it has ended.
Claims also enter the calculation at their reserved value rather than at what is eventually paid, meaning the insurer estimate of ultimate cost. An open claim with a large reserve affects the mod even if it eventually settles for far less, which makes claim reserve management a genuine cost issue rather than an administrative one.
Why It Reaches Beyond Insurance
The mod became more consequential than a premium adjustment because it is used as a qualification criterion.
Many construction contracts, particularly public work and large industrial projects, require bidders to have a mod below a threshold, commonly 1.00 or 1.25. A contractor above the threshold cannot bid regardless of price or capability.
That converts safety performance from a cost into a market access requirement, which is a considerably stronger incentive. A contractor with an elevated mod loses the ability to compete for work worth far more than the premium difference.
The Behaviour It Produces
The strength of the incentive produces both intended and unintended responses.
The intended response is genuine safety investment: training, equipment, hazard assessment, and supervision, all of which reduce frequency.
Another legitimate response is return to work programmes, bringing injured employees back in modified duties. This reduces indemnity costs and is generally better for recovery outcomes, so it is a rare case where the financial and human interests align cleanly.
The problematic response is claim suppression: discouraging employees from reporting injuries, treating injuries informally, or pressuring workers not to seek medical care that would create a recordable claim.
Because the formula punishes frequency specifically, the incentive to prevent a claim being recorded is stronger than the incentive to prevent the underlying injury, and research on injury underreporting suggests the effect is real. That is the central criticism of experience rating and it follows directly from the design choice that makes the system work.
What an Employer Should Actually Do
The practical levers are narrow. Verify the classification codes, since misclassified payroll distorts both the premium and the expected losses used in the calculation. Review open claim reserves with the insurer, since stale reserves on claims unlikely to develop inflate the mod. Operate a return to work programme. And understand the timing, because a safety improvement made today affects the mod in two to three years.
The Bottom Line
The experience modification factor prices an employer against its own claims record, weighting frequency far above severity because frequency measures management and severity measures luck. Its use as a bid qualification threshold in construction gives it force well beyond the premium it adjusts. The same design that makes it an effective safety incentive is what creates the pressure to suppress claims, and that tension has never been resolved because removing the frequency weighting would remove the part that works.