How to calculate the average value in literal equivalent?

Question

Project management

We evaluate suppliers, once a month a supplier receives a rating (by KPI) in the literal equivalent of A, B, C or D. during the year, every month this estimate changes.
You need to display the average rating for the current moment. What should the formula look like?

Answer 1

Assign digital equivalents to all literal values, calculate the arithmetic mean, cut off the fractional part - convert back to a letter