Hello! I have a long list of banks. For every bank there is a rating. The
rating consits of letters and sometimes a plus or minus sign eg AA-, BBB+, A.
On another spreadsheet I have a table that consists of ratings and mean
default rates. The table looks like:

• AAA 0%
  AA 2%
  A 5%
  BBB 11,2%

What I want to do is to get the mean default rate for every bank based on
the rating. The problem is that the ratings for the banks have the plus/minus
also. Thus I want to ignore them and just pick the rating based on the
letters. Any help very much apreciated! Thanks alot!