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The following table summarizes the level within the fair value hierarchy used to determine the fair value of the Company's pension plan assets for its U.K. plans at December 31, 2019 and 2018: 

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Short-term investment funds consist primarily of funds with a maturity of 60 days or less and are valued at amortized cost which approximates fair value. 

Equity securities consist primarily of common collective funds (Level 2). Common collective funds are valued at the net asset value per share multiplied by the number of shares held as of the measurement date. 

Fixed income securities consist of money market funds, government securities, corporate bonds and debt securities, mortgage-backed securities and other common collective funds. Government securities are valued by third-party pricing sources and are valued daily in an active market (Level 1). Corporate bonds are valued using either the yields currently available on comparable securities of issuers with similar credit ratings or using a discounted cash flows approach that utilizes observable inputs, such as current yields of similar instruments, and includes adjustments for valuation adjustments from internal pricing models which use observable inputs such as issuer details, interest rates, yield curves, default rates and quoted prices for similar assets (Level 2). Mortgage-backed securities are valued by pricing service providers that use broker-dealer quotations or valuation estimates from their internal pricing models (Level 2). Other common collective funds are valued at the net asset value per share multiplied by the number of shares held as of the measurement date (Level 2). 

Alternative strategy funds valued at the net asset value per share multiplied by the number of shares held as of the measurement date (Level 2). Alternative strategy funds may include derivative instruments such as futures, forward contracts, options and swaps and are used to help manage risks. Derivative instruments are generally valued by the investment managers or in certain instances by third party pricing sources (Level 2) or may, due to the inherent uncertainty of valuation for those investments, differ significantly from the values that would have been used had a ready market for the investments existed, and the differences could be material (Level 3). 

Real estate funds are primarily property unit trusts whose values are primarily reported by the fund manager and are based on valuation of the underlying investments which include inputs such as cost, discounted cash flows, independent appraisals and market-based comparable data (Level 3). The fair values may, due to the inherent uncertainty of valuation for those investments, differ significantly from the values that would have been used had a ready market for the investments existed, and the differences could be material. 

