Total Skill Index
The Total Skill Index (or TSI) is an official measure of a player's abilities. The formula used for the calculation is unknown, but several somewhat successful attempts have been made in approximating the true function. It is rumoured that HT-Bjorn proclaimed he'll change the TSI calculation formula as soon as someone figures it out.
As a general rule, the higher the TSI, the better the player. However, there are problems with this theory. A player who is average in several skills (e.g. inadequate in defending, passing, and scoring, but with excellent stamina) will often have a higher TSI than a more specialized player (e.g. excellent defending, but everything else poor or lower). TSI also increases with training and it can also increase and decrease with form. Sometimes, TSI will drop even though form does not. This is an easy way of spotting that a background form change has occurred, and the form sub-level has decreased.
TSI for goalkeepers is a different matter. The higher TSI for a keeper, the better. Since a keeper only specializes in one skill, goalkeeping skill can be measured quite accurately based on their TSI and form (e.g. using http://www.hottrick.org/hattrick_keeper_tool.php).
Interestingly, goalkeepers typically have higher TSI at the same skill level than outfield players. A common theory is that both form and stamina have a multiplying effect on the TSI, whereas the other skills (for keepers, goaltending only) contribute as a sum of exponential functions.
For Goalkeepers an approximated formula for TSI calculation is:
TSI = [(0.025f+0.1)[10(s-1)]^(3.4)]/100
With f = Form and s = Skill (1 = disastrous, 2 = wretched, etc.)
For outfield players, the formula for TSI calculation is of the following form:
TSI = sqrt(f)*sqrt(st)*(A*[pl^3+d^3+sc^3]+B*w^3+C*pa^3)^2
With f = Form (1 = disastrous, 2 = wretched, etc.
With st = Stamina skill
With pl = Playmaking skill
With d = Defending skill
With sc = Scoring skill
With w = Winger skill
With pa = Passing skill
With A,B,C being fixed constants (up to you to find them ;-))
- A very useful TSI Calculator for Hattrick Players can be found in Hattrick TSI Calculator by Chechesa