Moody's Investors Service last week said it would recalibrate its existing global scale ratings on taxable municipal bond issues and muni issuers' obligations under swap transactions in a move that will result in lower ratings in some, but not all, cases.

The rating agency has put 23 of the 28 outstanding global scale ratings under review. Three of the other five have already been recalibrated, and two are already under review, Moody's said.

A team from across the agency will work to recalibrate the ratings so that they are "comparable to ratings assigned to other issuers of similar credit risk."

Moody's also has plans to move all its municipal credits to the global ratings scale. Although it delayed that migration in October amid the credit market turmoil, it's proceeding with the recalibration of the muni credits that are already on the global scale.

Moody's said it expects to complete this group of recalibrations within the next few weeks. Last September it said it planned on reviewing the global scale ratings in the context of its shift of all its municipal credits to a global scale.

All three rating agencies came under pressure last year from politicians and others who said raters used dual-rating scales that systemically rated municipal credits lower than corporate credits with similar or higher rates of default. Critics claim the agencies' municipal scales led to increased borrowing costs through higher interest rates and the need for bond insurance.

Fitch Ratings last year released its exposure draft about moving to a single scale, but deferred its final decision on a change in October amid the global credit crisis. It said it plans to revisit the decision during the first quarter of this year.

Standard & Poor's says it has always had just one rating scale, but last year upgraded more than 2,000 municipal credits, citing the results of a recent default study.

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