A homeowner and a shopkeeper both buy fire coverage, but they are not the same risk. The exposure base is the ruler you use to measure how much risk each one brings, so the rate you charge scales with it.
An exposure base is the basic unit that measures a policy's exposure to loss. One unit is one exposure. Auto uses the car-year. Workers compensation uses $100 of payroll. Homeowners uses the house-year. General liability often uses sales or receipts.
The exposure base answers one question: as this quantity doubles, does expected loss roughly double? If yes, charging a rate per exposure unit produces fair, risk-sensitive premiums.
KEY: A strong exposure base satisfies three tests: it is proportional to expected loss, it is practical, and it respects historical precedent.
Proportional to expected loss. The base should vary directly with the risk. Payroll works for workers compensation because more payroll means more workers and more hours, so more injury exposure. A poor base moves independently of loss.
Common mistakes
- Pairing losses with the wrong exposure measure. Occurrence losses belong with earned exposures, not written. Using written exposures with earned losses distorts the pure premium.
- Choosing a base that is not proportional to loss. Headcount for workers compensation ignores wage differences; two firms with equal headcount but very different payrolls carry very different risk.
- Treating a rating variable as the base. In homeowners, amount of insurance moves expected loss, but house-years is more directly proportional, so amount of insurance belongs among the rating variables and not in the denominator.
Bottom line
- An exposure base is the unit of risk that premium is charged per (car-year, $100 of payroll, house-year, sales revenue).
- A good exposure base is proportional to expected loss, practical to obtain and verify, and consistent with historical precedent.
- Rate equals pure premium plus fixed expense, divided by one minus variable expense minus profit; the exposure base is the denominator that turns total losses into a per-unit rate.
- Pure premium equals losses and LAE divided by exposures; frequency equals claim count divided by exposures.
Exam shortcut
When a question names an exposure base, silently run the three tests: proportional to loss, practical to verify, and historically consistent. The base that fails one is the wrong answer. When several factors move expected loss, pick the one with the most direct proportional link as the base and push the rest into rating variables. That is why homeowners uses house-years, not amount of insurance.
The full lesson (about 2,012 words, 13 min read) adds 2 worked examples, all 7 common mistakes, a self-check, free in the app.
Learning objectives
- A1
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