Underwriting income indicates the performance of the Company's core underwriting function, excluding revenues and expenses such as net investment income (loss), other income, finance expenses, net realized and unrealized gains (losses) on investments, foreign exchange gains (losses) and transaction expenses. The Company believes the reporting of underwriting income enhances the understanding of our results by highlighting the underlying profitability of the Company's core insurance and reinsurance business. Underwriting profitability is influenced significantly by earned premium growth, adequacy of the Company's pricing and loss frequency and severity.
Underwriting profitability over time is also influenced by the Company's underwriting discipline, which seeks to manage exposure to loss through favorable risk selection and diversification, its management of claims, its use of reinsurance and its ability to manage its expense ratio, which it accomplishes through its management of acquisition costs and other underwriting expenses. The Company believes that underwriting income provides investors with a valuable measure of profitability derived from underwriting activities.
Managed gross premiums written represents gross premiums written by the Company and its operating affiliates. Managed gross premiums written differs from total gross premiums written, which the Company believes is the most directly comparable GAAP measure, due to the inclusion of premiums written on behalf of the Company's operating affiliates, AlphaCat Re 2011, Ltd. and AlphaCat Re 2012, Ltd., which are accounted for under the equity method of accounting.
Annualized net operating return on average equity is presented in the section above entitled “Net Operating Income (Loss), Net Operating Income (Loss) per share and Annualized Net Operating Return on Average Equity.” A reconciliation of diluted book value per common share to book value per common share, the most comparable U.S. GAAP financial measure, is presented in the section above entitled “Book Value Per Common Share and Diluted Book Value Per Common Share.” Net operating income (loss) is calculated based on net income (loss) excluding net realized gains (losses) on investments, net unrealized gains (losses) on investments, gains (losses) arising from translation of non-US$ denominated balances and non-recurring items. Realized gains (losses) from the sale of investments are driven by the timing of the disposition of investments, not by our operating performance. Gains (losses) arising from translation of non-US$ denominated balances are unrelated to our underlying business. Net operating income (loss) available (attributable) to Validus is defined as net operating income (loss) as defined above, but excluding income (loss) available (attributable) to noncontrolling interest.