Table 2: The Observed Transition Probability Matrices are Presented for the Time Intervals from Baseline to 18-month and from 18-month to 36-month. A Root Surface that has a One-Surface Lesion can Remain as a Root Surface with a One-Surface lesion (1) or Progress to Develop a 2-surface (2), 3-surface (3), or 4-surface lesion (4), or be missing (9)