Total Skill Index
From Hattrick
The Total Skill Index (or TSI) is a an official measure of a player's abilities. The formula by which it is calculated is unknown, but several attempts have been successful in approximating the true function.
A common theory is that both form and stamina have a multiplying effect on the TSI, when the other skills (for keepers, goaltending only) contribute as a sum of exponential functions.
Set pieces skill doesn't affact the TSI.
