Merrill Lynch is altering the methodology by which it incorporates credit rating information into its Global Bond Indices.

The investment firm said Thursday the new methodology will be based on the ratings of Fitch, Moody’s and S&P, and will result in adjusted ratings on about 2,300 of the 18,818 securities in the Global Broad Market, Global High Yield and Global Emerging Market Indices. Merrill says that the vast majority of those will move one rating grade higher. Only 17 bonds will fall below investment grade as a result of the new algorithm and none move from below investment grade to investment grade.

Previously, the algorithm was just based on ratings from Moody’s and S&P. For Canadian credit markets only, Fitch will be replaced by DBRS.

Also, Merrill is using new selection criteria for several indicies: U.S. High Grade Indices, Euro High Grade Indices, Global High Yield and Emerging Markets Indices, and U.S. Municipal Securities Indices. The High Yield 175 Index will be converted to a rules-based index with selection criteria designed to closely emulate the risk structure of the U.S. High Yield Master II Index. And, the determination of qualifying countries in the various global sovereign indices will become rules based.

Merrill Lynch first proposed the inclusion of Fitch in its ratings algorithm in a research report published on July 8. The report said, “with their significantly expanded coverage- Fitch now rates a high percentage of Merrill Lynch Global Index constituents and is increasingly becoming a referenced standard in portfolio policy guidelines.” Based on investor feedback during the comment period following the release of the report, the proposed changes were announced today, and will become effective on Dec. 31.

According to the Merrill Lynch report, “it is worth noting that of the 2,300 bonds whose rating will change under the new methodology, Moody’s and S&P ratings match only 5% of the time whereas Fitch matches either Moody’s or S&P 65% of the time.”