Standard & Poor's changed the outlook on a number of bonds issued by public housing agencies guaranteed by mortgage-backed securities that are secured by the federal government. Analysts lowered the outlook on the bonds to negative from stable because the debt has mortgage revenue invested in short-term instruments guaranteed by the U.S. government. On April 18, Standard & Poor's revised its outlook on the U.S. to negative from stable, while affirming the triple-A rating of the world's largest economy. Analysts also said the rating could change within two years. Standard & Poor's also affirmed the triple-A rating on the bonds sold through public housing agencies, citing "strength of the guarantees supporting the mortgage payments, as well as the investments in which monthly mortgage payments are deposited to make semiannual bond payments." If S&P ultimately downgrades the rating of the U.S. government, analysts "likely would lower the rating" on the public housing bonds, as well. "The credit strength of these bonds is based solely on the mortgage-backed security and the short-term investments that are permitted to be in U.S. government obligations," analysts said. Write to Jason Philyaw.