What is loan grading migration?

Loan grading migration describes how loans move across risk rating grades from one period to the next. It is usually shown in a transition matrix, where rows represent the starting grade, columns represent the ending grade, and each cell shows the share of loans or balances that moved. Loans that stay on the diagonal kept their rating, while those above or below it were upgraded or downgraded.

Migration analysis gives management and boards a forward-looking view of portfolio quality. A rising share of downgrades from pass to special mention, for example, often precedes increases in classified assets and charge-offs. Migration data also support CECL and probability of default models, which may use historical transition rates to project losses.

Migration statistics are only as meaningful as the ratings behind them. If ratings are updated only at annual review or after a payment problem, migration appears late and in large jumps. Changes to the rating scale or definitions break comparability over time. Analysts should separate migration driven by a few large loans from broader trends.