Credit rating

Short and long-term ratings

A rating expresses the likelihood that the rated party will go into default within a given time horizon. In general, a time horizon of one year or under is considered short term, and anything above that is considered long term. In the past institutional investors preferred to consider long-term ratings. Nowadays, short-term ratings are commonly used.

Corporate credit ratings

Credit ratings can address a corporation's financial instruments i.e. debt security such as a bond, but also the corporations itself. Ratings are assigned by credit rating agencies, the largest of which are Standard & Poor's, Moody's and Fitch Ratings. They use letter designations such as A, B, C. Higher grades are intended to represent a lower probability of default.

Agencies do not attach a hard number of probability of default to each grade, preferring descriptive definitions such as: "the obligor's capacity to meet its financial commitment on the obligation is extremely strong," or "less vulnerable to non-payment than other speculative issues…" (Standard and Poors' definition of an AAA-rated and a BB-rated bond respectively). However, some studies have estimated the average risk and reward of bonds by rating. One study by Moody's claimed that over a "5-year time horizon" bonds it gave its highest rating (Aaa) to had a "cumulative default rate" of 0.18%, the next highest (Aa2) 0.28%, the next (Baa2) 2.11%, 8.82% for the next (Ba2), and 31.24% for the lowest it studied (B2). (See "Default rate" in "Estimated spreads and default rates by rating grade" table to right.) Over a longer period, it stated "the order is by and large, but not exactly, preserved".

Another study in Journal of Finance calculated the additional interest rate or "spread" corporate bonds pay over that of "riskless" US Treasury bonds, according to the bonds' rating. (See "Basis point spread" in table to right.) Looking at rated bonds for 1973–89, the authors found a AAA-rated bond paid 43 "basis points" (or 43/100 of a percentage point) over a US Treasury bond (so that it would yield 3.43% if the Treasury yielded 3.00%). A CCC-rated "junk" (or speculative) bond, on the other hand, paid over 7% (724 basis points) more than a Treasury bond on average over that period.

Different rating agencies may use variations of an alphabetical combination of lowercase and uppercase letters, with either plus or minus signs or numbers added to further fine-tune the rating (see colored chart). The Standard & Poor's rating scale uses uppercase letters and pluses and minuses. The Moody's rating system uses numbers and lowercase letters as well as uppercase.

While Moody's, S&P and Fitch Ratings control approximately 95% of the credit ratings business, they are not the only rating agencies. DBRS's long-term ratings scale is somewhat similar to Standard & Poor's and Fitch Ratings with the words high and low replacing the + and −. It goes as follows, from excellent to poor: AAA, AA (high), AA, AA (low), A (high), A, A (low), BBB (high), BBB, BBB (low), BB (high), BB, BB (low), B (high), B, B (low), CCC (high), CCC, CCC (low), CC (high), CC, CC (low), C (high), C, C (low) and D. The short-term ratings often map to long-term ratings though there is room for exceptions at the high or low side of each equivalent.

S&P, Moody's, Fitch and DBRS are the only four ratings agencies that are recognized by the European Central Bank (ECB) for determining collateral requirements for banks to borrow from the central bank. The ECB uses a first, best rule among the four agencies that have the designated ECAI status, which means that it takes the highest rating among the four agencies – S&P, Moody's, Fitch and DBRS – to determine haircuts and collateral requirements for borrowing. Ratings in Europe have been under close scrutiny, particularly the highest ratings given to countries like Spain, Ireland and Italy, because they affect how much banks can borrow against sovereign debt they hold.

A. M. Best rates from excellent to poor in the following manner: A++, A+, A, A−, B++, B+, B, B−, C++, C+, C, C−, D, E, F, and S. The CTRISKS rating system is as follows: CT3A, CT2A, CT1A, CT3B, CT2B, CT1B, CT3C, CT2C and CT1C. All these CTRISKS grades are mapped to one-year probability of default.

Under the EU Credit Rating Agency Regulation (CRAR), the European Banking Authority has developed a series of mapping tables that map ratings to the "Credit Quality Steps" (CQS) as set out in regulatory capital rules and map the CQS to short run and long run benchmark default rates. These are provided in the table below: