Why the Period Basis Matters
A loss summary is not fully described by its total or its valuation date. It also needs a period basis: a rule for assigning claims or loss transactions to periods. The same claim can belong to one accident year, a different policy year, and a different report year. Payments on that claim may appear in still other calendar years.
Choosing the basis is therefore a question about what the summary is intended to describe. Is it organized by when the loss event occurred, when the policy began, when the claim was reported, or when a payment or accounting change was recorded? Those choices are not interchangeable. A sound comparison states both the basis and the loss measure being summarized.
The date rules are simple, but the unit being grouped matters. Accident-, policy-, and report-year summaries commonly group claim records into cohorts using a claim attribute. Calendar-year summaries commonly group payment transactions by payment date; a calendar-year incurred measure instead reflects paid losses and changes in unpaid loss reserves. A summary must say which of these it uses.
Compare the Assignment Rules
| Basis | Assignment rule for a claim or activity | Question it can help answer |
|---|---|---|
| Accident year | Year of occurrence | What losses arose from occurrences in this year? |
| Policy year | Year the policy became effective | What losses are associated with policies that began in this year? |
| Report year | Year the claim was first reported | What losses are associated with claims first reported in this year? |
| Calendar year, paid | Year a payment transaction was made | How much was paid during this accounting year? |
| Calendar year, incurred | Year in which paid losses and reserve changes are recognized | What loss cost was recognized during this accounting year? |
A claim’s occurrence, policy effective, and first-report dates are claim attributes. A payment date belongs to a particular transaction, not to the claim as a whole. A single claim can have multiple payments in different calendar years. For a consistent scope of loss and reserves, calendar-year incurred loss is commonly expressed as calendar-year paid loss plus the change in unpaid loss reserves from the start to the end of the year. It is not simply the total estimate of every claim whose occurrence date falls within that year.
Working with Claim-Cohort Summaries
For accident-, policy-, or report-year summaries, first establish the relevant date for each claim. Then assign the entire claim record to the period containing that date. If the summary uses incurred estimates, the estimates should be taken at a stated common valuation date; changing the valuation date changes the estimates even if the period assignment stays the same.
Worked Example: Reassigning the Same Claim Estimates
A hypothetical insurer reviews three claims as of December 31, 2025. The incurred estimates at that date are $12,000, $8,000, and $5,000. The claim records are:
| Claim | Occurrence date | Policy effective date | First report date | Incurred estimate |
|---|---|---|---|---|
| A | December 20, 2024 | July 1, 2024 | January 5, 2025 | $12,000 |
| B | February 2, 2025 | January 1, 2025 | February 3, 2025 | $8,000 |
| C | December 30, 2024 | January 1, 2025 | January 2, 2025 | $5,000 |
Method. Keep the valuation date and estimates fixed, and assign each claim by the date specified for the selected basis.
Accident year. Claims A and C occurred in 2024, so the 2024 total is $12,000 + $5,000 = $17,000. Claim B occurred in 2025, so the 2025 total is $8,000.
Policy year. Claim A’s policy began in 2024, giving a 2024 total of $12,000. The policies for B and C began in 2025, giving a 2025 total of $8,000 + $5,000 = $13,000.
Report year. All three claims were first reported in 2025, so the 2025 report-year total is $12,000 + $8,000 + $5,000 = $25,000. There are no listed claims reported in 2024.
Check. Each basis assigns all three claims exactly once, and each set of period totals sums to $25,000. The totals differ by year because the assignment dates differ, not because the underlying estimates changed. The common mistake is to call the $25,000 total a 2024 accident-year result because two occurrences were in 2024; the correction is to assign each claim using the date required by the stated basis.
Calendar-Year Activity Is Not a Claim Cohort
A calendar-year paid summary follows payment transactions. It can include payments on old claims and exclude claims that occurred during the year but have not yet been paid. To construct it, select the transactions whose payment dates fall within the calendar year and add their amounts. Do not assign a claim’s entire lifetime total to the year of its first payment.
Worked Example: Calendar-Year Payments Versus Accident-Year Payments
A hypothetical book has the following paid transactions through December 31, 2025:
| Claim | Occurrence date | Payment date | Payment |
|---|---|---|---|
| P | December 2024 | December 2024 | $2,000 |
| P | December 2024 | February 2025 | $3,000 |
| Q | March 2025 | June 2025 | $4,000 |
| R | November 2024 | January 2026 | $5,000 |
Method. For a calendar-year paid total, filter by payment date. For an accident-year paid total through the stated valuation date, filter to payments made by that date, then group those payments by the associated claim’s occurrence year.
Calendar-year 2025 paid. The February and June 2025 transactions qualify: $3,000 + $4,000 = $7,000. P’s $2,000 payment belongs to 2024, and R’s $5,000 payment belongs to 2026.
Accident-year paid through December 31, 2025. Claim P’s two payments made by the valuation date total $2,000 + $3,000 = $5,000 and belong to accident year 2024. Claim R has no payment by that date. The accident-year 2024 total is therefore $5,000. Claim Q’s $4,000 belongs to accident year 2025, making that accident-year paid total $4,000.
Check. The 2025 calendar-year total is $7,000, while the two accident-year totals through the valuation date sum to $5,000 + $4,000 = $9,000. The difference is expected: the calendar-year calculation includes only payments made in 2025, whereas the accident-year calculation includes all payments made through 2025 and assigns them by occurrence year. The common mistake is to put both of P’s payments into calendar year 2025 because they concern one claim; only the February payment was made in 2025.
A Basis-Selection Rule and Its Justification
A useful way to formalize a claim-cohort summary is to let \(L_j\) be the chosen loss measure for claim \(j\), measured at a stated valuation date, and let \(D_j\) be the date used by the selected basis. The total for period \(t\) is the sum of the claim measures whose assignment date falls in that period.
Here, \(\mathbf{1}(D_j \in t)\) equals 1 when claim \(j\)'s assignment date lies in period \(t\), and 0 otherwise. For accident year, \(D_j\) is the occurrence date; for policy year, it is the policy effective date; for report year, it is the first report date.
This expression follows directly from the grouping rule: each claim contributes its selected loss measure if it meets the period condition, and contributes zero otherwise. If the periods do not overlap and cover all assignment dates, each claim is counted once across the full set of periods. But a particular period can have a different total under each basis because the indicator condition changes. The formula does not turn a calendar-year payment transaction into a claim-cohort assignment; calendar-year paid summaries select transactions by their own payment dates.
Decide whether the intended summary concerns occurrence cohorts, policy cohorts, report cohorts, or activity recorded during an accounting year.
Specify paid, incurred, or another defined measure. For incurred estimates, state the valuation date; for calendar-year incurred loss, state the accounting treatment of reserve changes.
Use the matching claim date or transaction date for every record. Do not substitute a nearby date merely because it is easier to extract.
Confirm that records were neither omitted nor counted twice, and that the period totals reconcile to the selected records and measure.
Selecting the Basis for a Stated Purpose
The best basis is the one that matches the question, not the one that happens to produce the most convenient total. If the question asks about occurrences in a year, accident year is the direct choice. If it asks about policies that began in a year, use policy year. If it asks about claims first reported in a year, use report year. If it asks about payments made during a year, use calendar-year paid.
The basis also affects interpretation. A recent accident year can have claims that have not all been reported or fully settled by the valuation date. A policy-year cohort can contain occurrences in more than one calendar year because policy terms may cross calendar boundaries. A report-year cohort can contain claims arising from earlier occurrences. These are consequences of the assignment rules, not reasons to relabel the data.
Comparisons require particular care. Two summaries can be compared meaningfully only when the basis, loss measure, scope, and valuation date are understood. For example, a calendar-year paid amount and an accident-year incurred estimate answer different questions. Their numerical difference is not, by itself, evidence of a data error.
Worked Example: Choosing Between Policy Year and Accident Year
A hypothetical insurer wants to summarize incurred estimates as of December 31, 2025. Its stated question is: “How much incurred loss is associated with policies that began in each year?” The claim file contains:
| Claim | Policy effective date | Occurrence date | Incurred estimate |
|---|---|---|---|
| A | December 15, 2023 | January 10, 2024 | $6,000 |
| B | December 20, 2023 | December 28, 2023 | $4,000 |
| C | January 5, 2024 | December 30, 2023 | $3,000 |
| D | November 1, 2024 | January 2, 2025 | $7,000 |
Choose the basis. The question explicitly refers to policies that began in each year, so policy year is the appropriate basis. The common valuation date means the estimates are compared at the same point in time.
Calculate policy-year totals. A and B have 2023 effective dates, giving $6,000 + $4,000 = $10,000 for policy year 2023. C and D have 2024 effective dates, giving $3,000 + $7,000 = $10,000 for policy year 2024.
Contrast with accident year. B and C occurred in 2023, giving $4,000 + $3,000 = $7,000 for accident year 2023. A occurred in 2024, giving $6,000 for accident year 2024. D occurred in 2025, giving $7,000 for accident year 2025.
Check. Both groupings sum to $20,000 across all listed claims: $10,000 + $10,000 by policy year and $7,000 + $6,000 + $7,000 by accident year. The policy-year totals answer the stated question; the accident-year totals do not, even though they use the same estimates. A common mistake is to assign A to 2023 because its policy began then when calculating accident year. The correction is to use A’s 2024 occurrence date for accident-year grouping.
Common Mistakes
- Using the wrong date. Policy year is determined by policy effective date, not occurrence date; report year is determined by first report date, not payment date.
- Treating calendar year as a claim cohort. A calendar-year paid total selects payments made in the year. It does not collect every payment on claims that happened in that year.
- Mixing measures or valuation dates. A paid total and an incurred estimate are different measures. Claim-level incurred estimates also need a common valuation date for a consistent comparison.
- Assuming period totals must match. Different bases can produce different totals for a particular year because they assign claims differently. Reconcile the grand total only over the same complete set of records and the same measure.
- Choosing a basis before reading the question carefully. State what the requested period represents, then apply that basis consistently.
Check Your Understanding
For each problem, identify the requested basis before calculating. Treat all stated claim amounts as measured on the same stated basis and valuation date.
- A claim occurred on December 29, 2024, was reported on January 3, 2025, and has an incurred estimate of $2,400. In which accident year and report year should it appear, and what amount does it contribute to each corresponding year’s total? (Answer: accident year 2024: $2,400; report year 2025: $2,400)
- Two claims have incurred estimates of $3,500 and $1,500. Their policy effective dates are December 2023 and January 2024, respectively; both occurred in 2024. Find the 2023 and 2024 policy-year totals and the 2024 accident-year total. (Answer: policy year 2023: $3,500; policy year 2024: $1,500; accident year 2024: $5,000)
- A claim has payments of $900 on November 15, 2024, and $1,100 on February 10, 2025. Find its contribution to calendar-year 2025 paid loss and to accident-year paid loss through December 31, 2025, if the occurrence was in 2023. (Answer: calendar-year 2025 paid: $1,100; accident-year 2023 paid through December 31, 2025: $2,000)
- Claims X and Y each have a $4,000 incurred estimate at the same valuation date. X occurred in 2024, was reported in 2025, and has a policy effective date in 2023. Y occurred in 2025, was reported in 2024, and has a policy effective date in 2024. Find the 2025 report-year total and the 2024 accident-year total. (Answer: report year 2025: $4,000; accident year 2024: $4,000)
- A calendar-year accounting summary reports $6,000 paid during 2025, with unpaid loss reserves of $9,000 at the start of 2025 and $11,000 at year-end. Using calendar-year incurred loss equal to paid loss plus the change in unpaid reserves, calculate the incurred amount. (Answer: $8,000)