Is there a standardized way of transforming the ratings of any of the major ratings agencies (S&P, Moody's, Fitch) to a numerical value. Ideally, it might be possible to create a similar scale for all of them. I just found some academic papers, which just assigned numerical values, i.e. 1 for AAA and 22 for D. Just interested if there are conventions on how to convert the ratings. Additionally, it would help comparing the impact of ratings in empirical studies.
"Mapping of Standard & Poor’s Ratings Services’credit assessments under the Standardised Approach" - Link to Report In their approach the mapping is the following: