The older a fleet of one aircraft type, the less often it grows and the more often it shrinks, though at most ages it is likeliest to stay as it is. This describes whole fleets, not one aircraft's chance of retiring.
What this shows: The chart plots P(Keep), P(Grow) and P(Shrink) on a vertical axis of Probability.
Conditions and method
Status: exploratory (this models how a carrier grows, keeps or shrinks its fleet of an aircraft type, not an aircraft’s chance of retirement, and the age at which the odds tip is read off a coarse age grid with no interval around it).
The age at which the odds tip is where the fitted probability of shrinking the fleet first reaches that of growing it. The model’s age grid first shows the shrink probability at or above the grow probability at age 20; interpolating between the grid points at 15 and 20 puts the crossing at 16. It is a single point read off the fitted curves and has no interval.
The model describes a carrier’s choice to grow, keep or shrink its fleet of one aircraft type from one year to the next (2,138 carrier-type-years, 451 growing and 392 shrinking), with the average age of that fleet among its drivers. It is not the chance that an individual aircraft of a given age is retired. Its pseudo R-squared is 0.07.