A credit risk manager is in charge of credit risk analysis of large corporates at Bank XYZ. The 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 categories | Description |

|-------------------|-------------|

| 1 | High investment grade |

| 2 | Mid investment grade |

| 3 | Low investment grade |

| 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?