Why the Period Definition Comes First
A premium transaction can have several relevant dates: when the transaction is recorded, when a change takes effect, and when the policy term begins. Those dates can fall in different years. As a result, the same transaction can belong to one experience period under a transaction-date definition and another under a policy-year definition.
The first step is therefore not arithmetic. It is to identify what the experience period is defined to measure. A calendar-year written-premium total may use transactions recorded during the calendar year. A policy-year total may instead collect transactions attached to policies whose terms began during that policy year. Earned premium is different again: it is allocated to the period in which the coverage is provided, according to the specified earning method.
For a written-premium measure, apply the date rule given in the problem or data specification. If it defines calendar-year written premium by transaction date, include each transaction recorded in the calendar year, even when the transaction’s effective date is in another year. If the measure is policy-year written premium, first identify policies by their term effective dates, then collect the specified transactions for those policies. Neither rule should be silently substituted for the other.
For earned premium, the key question is how much of the premium corresponds to coverage inside the experience period. When straight-line daily earning is specified, a premium amount is earned uniformly over its stated coverage interval. Only the days that overlap the experience period contribute to that period’s earned premium. Other earning methods can allocate premium differently, so the specified method must be followed.
Assigning Written Premium
Suppose a hypothetical insurer’s calendar-year written-premium definition assigns each transaction to the year in which it is recorded. Premium changes are signed amounts: additional premium is positive, and return premium is negative. A transaction’s effective date may explain what coverage changed, but it does not replace the stated recording-date rule.
Worked Example: Calendar-Year Written Premium Across a Year Boundary
A hypothetical insurer defines calendar-year written premium by transaction recording date. The following transactions relate to one policy. Classify them into calendar years and calculate each year’s total.
| Transaction | Premium change | Effective date | Recorded date |
|---|---|---|---|
| New business | $1,000 | January 1, 2024 | December 30, 2024 |
| Endorsement charge | $180 | December 20, 2024 | January 3, 2025 |
| Return premium | −$75 | December 31, 2024 | December 29, 2024 |
| Endorsement charge | $120 | July 1, 2025 | July 2, 2025 |
Method. Use the recording date for each transaction, as required by the stated calendar-year written-premium definition.
The 2024 transactions were the $1,000 new-business charge and the $75 return premium:
The $180 endorsement was recorded in 2025, despite taking effect in 2024. The $120 endorsement was also recorded in 2025. Therefore:
Check. The two year totals sum to $1,225, which equals the signed total of all transactions: $1,000+$180−$75+$120=$1,225. A common mistake would be to put the $180 in 2024 because its effective date is in 2024. That would give 2024 written premium of $1,105 and 2025 written premium of $120, which is inconsistent with the stated recording-date definition.
Grouping Transactions by Policy Year
A policy-year experience period groups policies by the year their policy terms begin. Transactions are then associated with the applicable policy-year cohort, rather than automatically assigned to the year in which they are recorded. An endorsement recorded after year-end can therefore belong to the prior policy-year cohort if it relates to a policy that began in that cohort. Conversely, a transaction recorded before a policy’s start date can belong to the policy year in which that policy begins.
This requires a reliable link between a transaction and its policy. For a renewal, treat the renewed term according to the stated definition: if it is a new policy term, its effective date determines its policy-year cohort. Do not infer a cohort from the billing date or transaction date when the specified rule is based on policy effective dates.
Worked Example: Assigning Transactions to Policy-Year Cohorts
A hypothetical insurer defines policy-year written premium by the effective year of the policy term. All listed transactions belong to the policy shown. Determine the premium assigned to the 2024 and 2025 policy-year cohorts.
| Policy | Policy effective date | Transaction | Premium change | Recorded date |
|---|---|---|---|---|
| A | November 1, 2024 | New business | $900 | October 28, 2024 |
| A | November 1, 2024 | Endorsement charge | $120 | February 10, 2025 |
| A | November 1, 2024 | Return premium | −$60 | June 2, 2025 |
| B | January 1, 2025 | New business | $1,400 | December 20, 2024 |
Method. Assign each policy to the year of its policy effective date, then sum its signed transactions in that cohort.
Policy A began in 2024, so all three of its transactions belong to the 2024 policy-year cohort:
Policy B began in 2025, so its $1,400 transaction belongs to the 2025 policy-year cohort, even though it was recorded in December 2024:
Check. The cohort totals sum to $2,360, equal to the signed sum of all four transactions: $900+$120−$60+$1,400=$2,360. The trap is to assign Policy B’s premium to 2024 because it was recorded then, or to move Policy A’s endorsement and return premium to 2025 because they were recorded then. Both choices ignore the policy-effective-year rule.
Assigning Earned Premium by Coverage Period
Earned premium is assigned according to the coverage interval and the stated earning method, not merely the date on which a transaction was recorded. Under straight-line daily earning, assume premium is earned uniformly per day over its specified coverage interval. If that interval overlaps an experience period for only part of its duration, only the overlapping days are assigned to that period.
The justification follows from the earning assumption. Uniform earning over \(N\) days means the amount earned per day is \(P/N\). The period contains \(n\) covered days, so its earned amount is \(n(P/N)=P(n/N)\). Days outside the coverage interval contribute nothing; days in the interval but outside the experience period are earned in a different period.
Worked Example: Calendar-Year Earned Premium From Overlapping Coverage
A hypothetical policy covers July 1, 2024 through June 30, 2025, inclusive. Its $1,200 premium is earned straight-line by day over the 365-day term. A separate endorsement adds $91 of premium for coverage from April 1 through June 30, 2025, inclusive. Calculate the earned premium in calendar year 2025. Assume no cancellation and no other premium changes.
Method. For each premium amount, count the days in its own coverage interval that fall in calendar year 2025. Then apply the straight-line daily formula.
The original policy’s 2025 coverage is January 1 through June 30: 31+28+31+30+31+30=181 days. The full policy term has 365 days. Its 2025 earned premium is:
The endorsement’s entire 91-day coverage interval falls in 2025: April has 30 days, May has 31, and June has 30. Thus all $91 is earned in 2025:
Adding the two components gives \(595.06849\ldots+91=686.06849\ldots\), or $686.07 rounded to the nearest cent.
Check. The original policy contributes less than its full $1,200 because 184 of its 365 days fall in 2024. The endorsement contributes its full $91 because all of its stated coverage days fall in 2025. The common error is to assign the entire original premium to 2024 because the policy began then, or to assign the endorsement only to the year it was recorded. Neither is the specified calendar-year earned-premium method.
A Reliable Classification Process
The date fields should be treated as distinct data, not as interchangeable versions of a single “premium date.” The following process helps prevent period-assignment errors.
Determine whether the measure is based on transaction recording date, policy effective year, coverage dates, or another explicitly stated convention.
For written premium, use the transaction or policy date required by the definition. For earned premium, identify the coverage interval and earning method.
For an inclusive daily interval, count covered days from the first date through the last date. Check how the stated convention treats boundary dates.
Keep return premiums negative, sum each period, and compare the combined totals with the transactions or earned amounts being classified.
Common Traps
- Substituting effective date for recording date. This changes calendar-year written premium when the definition is based on transaction recording.
- Substituting recording date for policy effective year. This can move a policy’s transactions into the wrong policy-year cohort.
- Treating written premium as earned premium. A transaction can be written in one period while its coverage is earned over several periods.
- Ignoring the sign of a return premium. A return premium reduces the relevant total; it is not added as a positive charge.
- Using the wrong day count. Under daily earning, count the overlap and the full stated coverage interval using the same inclusive-date convention. Do not assume every annual term has the same number of days.
- Mixing period definitions within one total. A total is not interpretable if some transactions are assigned by recording date and others by policy effective date without an explicit reason.
Check Your Understanding
For each problem, follow the stated period definition and keep the date fields separate.
- A hypothetical insurer defines 2026 calendar-year written premium by transaction recording date. A $640 charge is recorded December 31, 2025, and a −$90 return premium is recorded January 2, 2026. What amount is assigned to 2026? (Answer: −$90)
- A policy effective September 1, 2025 has a $700 initial premium, a $50 endorsement recorded in February 2026, and a −$20 return premium recorded in March 2026. Under a policy-year definition based on policy effective year, what is assigned to the 2025 policy-year cohort? (Answer: $730)
- A hypothetical policy has a 365-day term from July 1, 2025 through June 30, 2026, with $1,095 premium earned straight-line by day. How much is earned in calendar year 2026, which contains 181 days of the term? (Answer: $543.00)
- A hypothetical policy has $730 premium for a 365-day coverage interval from October 1, 2025 through September 30, 2026, earned straight-line by day. A calendar-year experience period is 2026. How much of this premium is earned in 2026? (Answer: $546.00)
- A policy effective December 15, 2025 has a $500 charge recorded December 10, 2025 and a $40 endorsement recorded January 5, 2026. If policy-year written premium is assigned by policy effective year and both transactions belong to this term, how much is assigned to the 2025 cohort? (Answer: $540)