Investments
All securities held by the Old-Age and Survivors Insurance (OASI) and Disability Insurance (DI) Trust Funds are issued by the Federal government. All of these securities are special issues—securities issued only to the trust funds. In the past, the trust funds also held marketable securities, which are available to the public.

Interest rates
Special-issue investments bear interest rates determined monthly by a formula specified by law. An interest rate is determined on the last business day of a month and applies to securities issued in the following month. Tables of such monthly interest rates provide rates back to 1937—the beginning of the Social Security program.

Frequency of interest payments
Interest on special-issue investments is paid semi-annually, at the end of June and the end of December. Because the trust funds hold no cash, investments are redeemed each month to pay for benefits and administrative expenses. When investments are redeemed, interest is paid. The amount of interest paid is used to offset the amount of investment redemptions.

Effective interest rates
Although new investments for each trust fund carry the same interest rate, amounts of income and outgo differ between funds, resulting in a different investment portfolio for each trust fund. An effective interest rate provides a measure of the rate of return on an investment portfolio. We calculate an effective interest rate by dividing interest earned on investments during a calendar year by the average level of investments during the year. A table of such effective rates provides rates back to 1940 by trust fund.