fixed-rate agency securities. (See Financial Condition below for further details of our portfolio composition as of December 31, 2012).
The following table summarizes recent prepayment trends for our portfolio and, for comparison, Fannie Mae 2011 30-year 4.0% fixed-rate generic mortgage-backed securities for fiscal year 2012.
Annualized Monthly Constant Prepayment Rates (1)
Fannie Mae 2011 30-year 4.0% fixed-rate MBS (2)
Weighted average actual one-month annualized CPR released at the beginning of the month based on securities held/outstanding as of the preceding month-end.
Summary of Critical Accounting Estimates
Our critical accounting estimates relate to the recognition of interest income and the fair value of our investments and derivatives. Certain of these items involve estimates that require management to make judgments that are subjective in nature. We rely on our Manager's experience and analysis of historical and current market data in order to arrive at what we believe to be reasonable estimates. Under different conditions, we could report materially different amounts based on such estimates. The remainder of our significant accounting policies are described in Note 2 to the consolidated financial statements included under Item 8 of this Annual Report on Form 10-K.
The effective yield on our agency securities is highly impacted by our estimate of future prepayments. We accrue interest income based on the outstanding principal amount of our investment securities and their contractual terms and we amortize or accrete premiums and discounts associated with the purchase of investment securities into interest income over the projected lives of our securities, including contractual payments and estimated prepayments, using the interest method. The weighted average cost basis of our securities as of December 31, 2012 was 105.6% of par value; therefore, faster actual or projected prepayments can have a meaningful negative impact, while slower actual or projected prepayments can have a meaningful positive impact, on our asset yields.
Future prepayment rates are difficult to predict and we rely on a third-party service provider and our Manager's experience and analysis of historical and current market data in order to arrive at what we believe to be reasonable estimates. Our third-party service provider 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 and, based on our Manager’s judgment, we may make adjustments to their estimates.
We review our actual and anticipated prepayment experience on at least a quarterly basis and effective yields are recalculated when differences arise between (i) our previously estimated future prepayments and (ii) actual prepayments to date plus current 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.
The most significant factor impacting prepayment rates on our securities is changes to long-term interest rates. Prepayment rates generally increase when interest rates fall and decrease when interest rates rise. However, there are a variety of other factors that may impact the rate of prepayments on our securities. Prepayments can also occur when borrowers sell the property and use the sale proceeds to prepay the mortgage as part of a physical relocation. In addition, changes to the GSE's underwriting standards, further modifications to existing U.S. Government sponsored programs such as HARP, or the implementation of new programs can have a significant impact on the rate of prepayments. Further, GSE buyouts of loans in imminent risk of default,