separate component of stockholders' equity. Upon the sale of a security, we determine the cost of the security and the amount of unrealized gains or losses to reclassify out of accumulated OCI into earnings based on the specific identification method.
Interest-only securities and inverse interest-only securities (collectively referred to as "interest-only securities") represent our right to receive a specified proportion of the contractual interest flows of specific agency CMO securities. Principal-only securities represent our right to receive the contractual principal flows of specific agency CMO securities. Interest and principal-only securities are measured at fair value through earnings in gain (loss) on derivative instruments and other securities, net in our consolidated statements of comprehensive income. Our investments in interest and principal-only securities are included in agency securities, at fair value on the accompanying consolidated balance sheets.
REIT equity securities represent investments in the common stock of other publicly traded mortgage REITs that invest predominantly in agency MBS. We designate our investments in REIT equity securities as trading securities and report them at fair value on the accompanying consolidated balance sheets.
We estimate the fair value of our agency securities based on valuations obtained from third-party pricing services and non-binding dealer quotes derived from common market pricing methods using observable (or "Level 2") inputs. Such methods incorporate, but are not limited to, reported trades and executable bid and asked prices for similar securities, benchmark interest rate curves, such as the spread to the U.S. Treasury rate and interest rate swap curves, convexity, duration and the underlying characteristics of the particular security, including coupon, periodic and life caps, rate reset period, issuer, additional credit support and expected life of the security. We estimate the fair value of our REIT equity securities based on a market approach using quoted market prices (or "Level 1" inputs). Refer to Note 8 for further discussion of fair value measurements.
We evaluate our agency securities for other-than-temporary impairment ("OTTI") on at least a quarterly basis. The determination of whether a security is other-than-temporarily impaired may involve judgments and assumptions based on subjective and objective factors. When a security is impaired, an OTTI is considered to have occurred if any one of the following three conditions exists as of the financial reporting date: (i) we intend to sell the security (that is, a decision has been made to sell the security), (ii) it is more likely than not that we will be required to sell the security before recovery of its amortized cost basis or (iii) we do not expect to recover the security's amortized cost basis, even if we do not intend to sell the security and it is not more likely than not that we will be required to sell the security. A general allowance for unidentified impairments in a portfolio of securities is not permitted.
Interest income is accrued based on the outstanding principal amount of the investment securities and their contractual terms. Premiums or discounts associated with the purchase of investment securities are amortized or accreted into interest income, respectively, over the projected lives of the securities, including contractual payments and estimated prepayments using the effective interest method in accordance with ASC Subtopic 310-20, Receivables—Nonrefundable Fees and Other Costs ("ASC 310-20").
We estimate long-term prepayment speeds of our agency securities using a third-party service and market data. The third-party service estimates prepayment speeds using models that incorporate the forward yield curve, current mortgage rates and mortgage rates of the outstanding loans, age and size of the outstanding loans, loan-to-value ratios, volatility and other factors. We review the prepayment speeds estimated by the third-party service and compare the results to market consensus prepayment speeds, if available. We also consider historical prepayment speeds and current market conditions to validate the reasonableness of the prepayment speeds estimated by the third-party service. As market conditions may change rapidly, we may make adjustments for different securities based on our Manager's judgment. Various market participants could use materially different assumptions.
Actual and anticipated prepayment experience is reviewed quarterly and effective yields are recalculated when differences arise between (i) our previously estimated future prepayments and (ii) the actual prepayments to date plus our currently estimated future prepayments. If the actual and estimated future prepayment experience differs from our prior estimate of prepayments, we are required to record an adjustment in the current period to the amortization or accretion of premiums and discounts for the cumulative difference in the effective yield through the reporting date.
We use a variety of derivative instruments to hedge a portion of our exposure to market risks, including interest rate risk, prepayment risk and extension risk. The objective of our risk management strategy is to reduce fluctuations in net book value over a range of interest rate scenarios. In particular, we attempt to mitigate the risk of the cost of our variable rate liabilities increasing during a period of rising interest rates. The principal instruments that we use are interest rate swaps and options to enter into interest rate swaps ("swaptions"). We also utilize forward contracts for the purchase or sale of agency MBS securities in the "to-be-announced" market on a generic pool basis ("TBA securities") and we utilize U.S. Treasury securities and U.S. Treasury