A risk manager is in possession of credit ratings provided by two rating agencies, X and Y, for 30 companies the manager oversees. The ratings are classified into four categories:

• Rating 1: High investment grade

• Rating 2: Mid investment grade

• Rating 3: Low investment grade

• Rating 4: Non-investment grade

The manager plots the rating categories from the two agencies as shown below:

[Image blocked: Corporate Ratings: Agency X vs. Agency Y]

Which of the following statistical measures could best help the manager approximate the link between rating categories from the two agencies?