I also want to know the equation to calculate training effectiveness. But probably this is not a simple logic that I can easily understand. Besides factors I know like age, market value, quality star levels, there may be other factors.

The fastest trainer of my level 15 team in this season is a 21 years old, low 5 star defender. His average training effectiveness in this season is the best in 3 seasons when star quality level are 5. Now he is the forth lowest market value player in the team (partially because he is one of oldest player). But his average training effectiveness is better than those younger, higher prized teammates in same star level.