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.
Obviously, the higher TSI, the better. But it isn't very accurate because an average all around player (inadequate in a few positions) would have a higher TSI than specialized players (excellent in only one). It increases with training and it can increase and decrease with form. Sometimes your TSI will drop even though your form does not. (That is a background form change for example when you go from a high passable to a low passable form level. It doesn't show up as a form change but you dropped a sublevel)
TSI for goalies is a different matter. The highest TSI for a goalie the better. Since a goalie only specializes in one skill it can be measured and you can see how close your goalie is to the next level by entering your TSI into http://www.hottrick.org/hattrick_keeper_tool.php. Also keepers have higher TSI than their specialized equavalents.
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 = ((0,025 * FORM + 0,1) * ( 10*(SKILL-1) ) ^ (3.4))/1000 With Form and Skill ranked from 1 (disastrous)