How is PAI calculated?

• A. PAI = Ya - Ya-1.
• B. PAI = (Ya2 - Ya1)/(a2 - a1).
• C. PAI = Ya2 - Ya1.
• D. PAI = MAI = Ya / a.

Answer: (B)

PAI measures how much growth occurs per year over a interval. It is the slope of the yield‑at‑age curve between two ages, so you take the increase in yield and divide by the length of the interval. The correct form is PAI = (Ya2 − Ya1) / (a2 − a1).

For example, if yield at age a1 is Ya1 and at a2 is Ya2, then PAI tells you the average annual increment during that period. This is different from MAI, which is Ya divided by age a (the average per year up to a point).

The other options either give a total increase (without dividing by the interval length) or a one-year change, or mix in MAI instead of the interval-averaged rate.