1. National Association of Insurance Commissioners (NAIC). Statement of Statutory Accounting Principles (SSAP) No. 53 – Property and Casualty Contracts-Premiums. This standard defines the accounting treatment for P&C premiums. Paragraph 9 explicitly states: "Earned premium shall be the result of premiums written or received during the period, plus the unearned premium reserve at the beginning of the period, less the unearned premium reserve at the end of the period."
2. Trieschmann, J. S., Hoyt, R. E., & Sommer, D. W. (2017). Risk Management and Insurance (13th ed.). Cengage Learning. In the chapter on "Insurance Company Operations," the financial statements section details this exact formula for calculating earned premiums as a fundamental component of an insurer's income statement. (See Chapter 7, Financial Operations of Insurers).
3. Pottier, S. W. (2016). The U.S. property-casualty insurance industry's financial performance and capacity. Risk Management and Insurance Review, 19(1), 103-125. https://doi.org/10.1111/rmir.12051. This academic review discusses key financial metrics for P&C insurers, where the calculation of earned premium (Premiums Written + Change in Unearned Premium Reserve) is a foundational concept for assessing underwriting profitability.