- y1
y_1, x_1 = survival time (days) from date of first hospital admission
- y2
y_1, x_1 = survival time (days) from date of first hospital admission
- x1
y_1, x_1 = survival time (days) from date of first hospital admission
- x2
y_1, x_1 = survival time (days) from date of first hospital admission