rating codes, agencies typically supplement the current assessment with indications of the chances for future upgrades or downgrades over the medium term. For example, Moody's designates an Outlook for a given rating as Positive (POS, likely to upgrade), Negative (NEG, likely to downgrade), Stable (STA, likely to remain unchanged), or Developing (DEV, contingent on some future event). Rating tier definitions Investment grade A bond is considered investment grade or IG if its credit rating is BBB− or higher by Fitch Ratings or S&P, or Baa3 or higher by Moody's, the so-called "Big Three" credit rating agencies. Generally they are bonds that are judged by the rating agency as likely enough to meet payment obligations that banks are allowed to invest in them. Ratings play a critical role in determining how much companies and other entities that issue debt, including sovereign governments, have to pay to access credit markets, i.e., the amount of interest they pay on their issued debt. The threshold between investment-grade and speculative-grade ratings has important market implications for issuers' borrowing costs. Bonds that are not rated as investment-grade bonds are known as high yield bonds or more derisively as junk bonds. The risks associated with investment-grade bonds (or investment-grade corporate debt) are considered significantly higher than those associated with first-class government bonds. The difference between rates for first-class government bonds and investment-grade bonds is called investment-grade spread. The range of this spread is an indicator of the market's belief in the stability of the economy. The higher these investment-grade spreads (or risk premiums) are, the weaker the economy is considered. Criticism Until the early 1970s, bond credit ratings agencies were paid for their work by investors who wanted impartial information on the credit worthiness of securities issuers and their particular offerings. Starting in the early 1970s, the "Big Three" ratings agencies (S&P, Moody's, and Fitch) began to receive payment for their work by the securities issuers for whom they issue those ratings, which has led to charges that these ratings agencies can no longer always be impartial when issuing ratings for those securities issuers. Securities issuers have been accused of "shopping" for the best ratings from these three ratings agencies, in order to attract investors, until at least one of the agencies delivers favorable ratings. This arrangement has been cited as one of the primary causes of the subprime mortgage crisis (which began in 2007), when some