External and Internal Ratings

After completing this reading you should be able to:

  • Describe external rating scales, the rating process, and the link between ratings and default.
  • Describe the impact of time horizon, economic cycle, industry, and geography on external ratings.
  • Explain the potential impact of ratings changes on bond and stock prices.
  • Compare external and internal ratings approaches.
  • Explain and compare the through-the-cycle and at-the-point internal ratings approaches.
  • Describe a ratings transition matrix and explain its uses.
  • Describe the process for and issues with building, calibrating, and backtesting an internal rating system.
  • Identify and describe the biases that may affect a rating system.

A Description of External Rating Scales and the Rating Process

An external rating scale is a scale used as an ordinal measure of risk. The highest grade on the scale represents the least risky investments, but as we move down the scale, the amount of risk gradually increases (safety decreases).

An issue-specific credit rating conveys information about a specific instrument, such as a zero-coupon bond issued by a corporate entity. An issuer-specific credit rating, on the other hand, conveys information about the entity behind an issue. The latter usually incorporates a lot more information about the issuer.

Here are S&P’s and Moody’s credit rating scores for long-term obligations:

frm-sp-moodys-ratingsEach successive move down the scale represents an increase in risk. In case of Moody’s ratings Baa and above are said to be investment-grade while those below this level are said to be non-investment-grade.

In the case of S&P’s, ratings BBB and above are investment-grade. All the others are non-investment-grade.

The Rating Process

The process leading up to the issuance of a credit rating follows certain steps. These are:

  1. A qualitative analysis of the company, including assessments of the quality of management and competitive aspects
  2. A quantitative analysis of financials such as ratio analysis
  3. A meeting with the firm’s management
  4. A meeting of the rating agency committee assigned to rating the firm
  5. A notification is sent to the rated firm detailing the assigned rating
  6. The rated firm has a window to appeal the assigned rating or offer new information
  7. The assigned rating is published

The Impact of Time Horizon, Economic Cycle, Industry, and Geography on External Ratings

Time Horizon:

The probability of default given any rating at the beginning of a cycle increases with the time horizon. Non-investment bonds are the worst hit. Their default probabilities can dramatically increase within a short time.

Economic Cycle:

Since ratings are generally produced with an eye on a long-term period, they must take into account any economic/industrial cycle on the horizon. Rating agencies make efforts to incorporate the effects associated with an economic cycle in their ratings. Although this practice is generally valid, it can lead to underestimation or overestimation of default if the predicted economic cycle doesn’t play out exactly as expected. Put precisely, the probability of default can be underestimated if an economic recession occurs, or overestimated if an economic boom occurs. In addition, the default rate of lower-grade bonds is correlated with the economic cycle, while the default rate of high-grade bonds is fairly stable.

Industry:

Two firms in different industries – say, banking and manufacturing – could have the same rating, but the probability of default may be higher for one of the firms than for the other. What does that mean? The implication here is that for a given rating category, default rates can vary from industry to industry. However, there’s little evidence to support the notion that geographic location has a similar effect.

The Impact of Ratings Changes on Bond and Stock Prices

Bonds:

There’s overwhelming evidence that a rating downgrade triggers a decrease in bond prices. In fact, bond prices sometimes decrease just because there’s a strong possibility of a downgrade. Anxious investors tend to sell bonds whose credit quality is declining.

A rating upgrade triggers an increase in bond prices, although there’s relatively less market evidence to support this conclusion.

Therefore, the underperformance of bonds whose credit quality has been downgraded is more statistically significant compared to the over-performance of bonds recently upgraded.

Stocks:

There’s moderate evidence to support the view that a rating downgrade will lead to a stock price decrease. A ratings upgrade, on the other hand, is somewhat likely to trigger an increase in bond prices.

In practice, the relationship between changes in rating and stock prices can be quite complex and will usually be heavily impacted by the reason behind the changes. Furthermore, downgrades tend to have more impact on the stock price compared to upgrades.

Comparing External and Internal Ratings Approaches

External ratings are produced by independent rating agencies and aim at revealing the financial stability of both lenders and borrowers. For example, Moody’s periodically releases ratings for big banks around the globe. Such ratings are important because banks usually rely on customer deposits and money raised through the issuance of various assets such as bonds to sustain lending. The funds raised this way create a pool of money that is then loaned to borrowers in smaller chunks. Thus, depositors and bond owners use such ratings to assess the riskiness of giving their money to the bank.

Sometimes, however, banks also need their own ratings so as to undertake an independent assessment of the creditworthiness of a specific borrower – either an individual or a corporate. That’s where internal credit ratings come in.

In modern times, internal credit ratings are usually developed based on the techniques used to develop external credit ratings. The same indicators are used, albeit with a few adjustments depending on whether the borrower is an individual or a corporate.

Some of the factors that have contributed to the increased sophistication of modern internal credit ratings are:

  1. The ever growing use of external credit rating agency language in financial markets
  2. Enforcement of capital requirements such as Basel II

Comparing the Through-the-cycle and At-the-point Internal Ratings Approaches

At-the-point Internal Ratings:

At-the-point internal ratings, also called point-in-time ratings, evaluate the current situation of a customer by taking into account both cyclical and permanent effects. As such, they are known to react promptly to changes in the customer’s current financial situation.

At-the-point ratings try to assess the customer’s quantitative financial data (e.g. balance sheet information), qualitative factors (e.g. quality of management), and information about the state of the economic cycle. Using statistical procedures such as scoring models, all that information is transformed into rating categories.

At-the-point internal ratings are only valid for the short-term or medium term, and that’s largely because they take into account cyclic information. They are usually valid for a period not exceeding one year.

Through-the-cycle Internal Ratings:

Through-the-cycle (ttc) internal ratings try to evaluate the permanent component of default risk. Unlike at-the-point ratings, they are said to be nearly independent of cyclical changes in the creditworthiness of the borrower. They are not affected by credit cycles, i.e. they are through-the-cycle. As a result, they are less volatile than at-the-point ratings and are valid for a much longer period (exceeding one year).

Advantages of ttc ratings include:

  1. They are much more stable over time compared to at-the-point ratings
  2. Because of their low volatility, ttc ratings help financial institutions to better manage customers. Too many rating changes necessitate changes in the way a bank handles a customer, including the products the bank is ready to offer.

One of the disadvantages of ttc ratings over at-the-point ratings is that they can at times be too conservative if the stress scenarios used to develop the rating are frequently materially different from the firm’s current condition. If the firm’s current condition is worse than the stress scenarios simulated, then the ratings may be too optimistic. In fact, ttc ratings have very low default prediction in the short-term.

Ratings Transition Matrices and Their Uses

A rating transition matrix gives the probability of a firm ending up in a certain rating category at some point in the future, given a specific starting point. The matrix, which is basically a table, uses historical data to show exactly how bonds that begin, say, a 5-year period with an Aa rating, change their rating status from one year to the next. Most matrices show one-year transition probabilities.

Transition matrices demonstrate that the higher the credit rating, the lower the probability of default.

The table below presents an example of a rating transition matrix according to S&P’s rating categories:

$$ One-year\quad transition\quad matrix $$

$$
\begin{array}{|l|ccccccc|}
\hline
Initial & {} & {} & {} & {} & {} & {} & {} \\
{} & Rating& at \quad year & end & {} & {} & {} & {} \\
Rating & {} & {} & {} & {} & {} & {} & {} \\ \hline
{} & AAA & AA & A & BBB & BB & B & CCC & Default \\ \hline
AAA & 90.81\% & 8.33\% & 0.68\% & 0.06\% & 0.12\% & 0.00\% & 0.00\% & 0.00\% \\
AA & 0.70\% & 90.65\% & 7.79\% & 0.64\% & 0.06\% & 0.14\% & 0.02\% & 0.00\% \\
A & 0.09\% & 2.27\% & 91.05\% & 5.52\% & 0.74\% & 0.26\% & 0.01\% & 0.06\% \\
BBB & 0.02\% & 0.33\% & 5.95\% & 86.93\% & 5.30\% & 1.17\% & 0.12\% & 0.18\% \\
BB & 0.03\% & 0.14\% & 0.67\% & 7.73\% & 80.53\% & 8.84\% & 1.00\% & 1.06\% \\
B & 0.00\% & 0.11\% & 0.24\% & 0.43\% & 6.48\% & 83.46\% & 4.07\% & 5.20\% \\
CCC & 0.22\% & 0.00\% & 0.22\% & 1.30\% & 2.38\% & 11.24\% & 64.86\% & 19.79\% \\ \hline
\end{array}
$$

Exam tips:

  • Each row corresponds to an initial rating
  • Each column corresponds to a rating at the end of 1 year. For example, a bond initially rated BB has a 8.84% chance of moving to a B rating by the end of the year.
  • The sum of the probabilities of all possible destinations, given an initial rating, is equal to 1 (100%)
  • You will need to recall the rules of probability from mathematics to come up with \(n\)-year transition probabilities, where \(n > 1\)
  • Credit ratings are their most stable over a one-year horizon. Stability decreases with longer horizons.

Building, Calibrating, and Backtesting an Internal Rating System

Building:

To build an internal rating system, banks try to replicate the methodology used by rating agency analysts. Such a methodology consists of identifying the most meaningful financial ratios and risk factors. After that, these ratios and factors are assigned weights such that the final rating estimate is close to what a rating agency analyst would come up with.

Weights attached to financial ratios or risk factors are either defined qualitatively following consultations with an agency analyst, or extracted using statistical techniques.

Calibrating:

Internal ratings have two main uses:

  1. Assessing the creditworthiness of a customer during the loan application process
  2. To determine the value of inputs used in the modeling of capital required as per the existing regulations, e.g. Basel II

For these reasons, internal ratings have to be calibrated. This involves establishing a link between the internal rating scale and tables displaying the cumulative probabilities of default. The timeline of such tables must capture all maturities, from, say, 1 year to 30 years. Sometimes, it may be necessary to build different transition matrices that are specific to the asset classes owned by the bank.

Backtesting:

Before linking default probabilities to internal ratings, backtesting of the current internal rating system is vital. The question is: Just how many years are needed to pull this off?

A historical sample of between 11 and 18 years is considered sufficient to test the validity of ratings.

Biases that May Affect a Rating System

$$
\begin{array}{|l|l|}
\hline
Bias & Description \\ \hline
Time\quad horizon\quad bias & Using\quad a\quad combination\quad of\quad at-the-point\quad and \\
{} & through-the-cycle\quad approaches\quad to\quad score\quad a\quad company \\ \hline
Information \quad bias & Assigning\quad ratings\quad based\quad on\quad insufficient\quad information \\ \hline
Homogeneity \quad bias & Inability\quad to\quad maintain\quad consistent\quad rating\quad methods \\ \hline
Principal-agent \quad bias & Ratings\quad developers\quad fail\quad to\quad act\quad in\quad the\quad best \\
{} & interests\quad of\quad the\quad management \\ \hline
Backtesting \quad bias & Incorrectly\quad linking\quad rating\quad system\quad to\quad default\quad rates \\ \hline
Distribution \quad bias & Modeling\quad the\quad probability\quad of\quad default\quad using\quad an \\
{} & inappropriate \quad distribution \\ \hline
Scale \quad bias & Producing\quad ratings\quad that\quad are\quad not\quad stable\quad with \\
{} & the \quad passage \quad of \quad time \\ \hline
\end{array}
$$

Questions

Question 1

You have been given the following one-year transition matrix:

$$
\begin{array}{|l|cccc|}
\hline
Rating \quad From & {} & Rating \quad To & {} & {} \\ \hline
{} & A & B & CCC & Default \\ \hline
A & 80\% & 10\% & 10\% & 0\% \\ \hline
B & 5\% & 85\% & 5\% & 5\% \\ \hline
CCC & 0\% & 10\% & 70\% & 20\% \\ \hline
\end{array}
$$

Determine the probability that a B –rated firm will default over a two-year period.

  1. 5%
  2. 4.25%
  3. 1%
  4. 5.04%

The correct answer is D.

Required probability = Sum of probabilities of all possible paths that could lead to a rating of D (default) after two years.

In other words, in how many ways can a B-rated firm default over a two year period? The following are the possible paths:

$$
\begin{array}{|l|l|}
\hline
Path & Probability \\ \hline
B\rightarrow default & 0.05 \\ \hline
B \rightarrow B\rightarrow default & 0.85 \times 0.05=0.0425 \\ \hline
B \rightarrow CCC \rightarrow default & 0.05 \times 0.20=0.01 \\ \hline
Total & 0.1025 \\ \hline
\end{array}
$$

 

Question 2

ABC Co., currently rated BBB, has an outstanding bond trading in the market. Suppose the company is upgraded to A. What will be the most likely effect on the bond’s price?

  1. Positive and stronger than the negative effect triggered by a bond downgrade
  2. Negative and stronger than the positive effect triggered by a bond downgrade
  3. Positive and weaker than the negative effect triggered by a bond downgrade
  4. Positive and as strong as the negative effect triggered by a bond downgrade

The correct answer is C.

Rating downgrades tend to have more impact on the stock price compared to upgrades. This can be explained by the fact that firms tend to release good news a lot more often than bad news, and thus the expectations among investors are generally positive. Negative news is usually unexpected and unanticipated, triggering a stronger downward effect.


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