About the average alpha of our portfolio

This week’s Barrons wrote that: “the average annual return of the Vanguard 500 Index mutual fund (ticker: VFINX) for the 18 years through May 31 was a robust 9.9%, for an aggregate total return of 446.9”.

As you can calculate from the graph the average annual return of our portfolio is 26,49%, but this is based on only 9 years.

So the average yearly alpha of our portfolio is 26,49-9,9=16,59%