I personally hate on-line tests.
1) To reveal one's type you need to see him/her, see his/her reaction to the questions - a person may be saying "Maths? Ha! I loved maths at school. I'm hand-and-glove with everything that involves using one's wits and logic!.. bla... bla..." - and squeesing his/her hands, griping teeth or otherwise showing how unpleasant it is to answer this question.
2) A person may judge about him/herself mistakenly
3) A person may have a mask
4) efery new information must be checked many times before making a conclusion - on-line tests never offer that.
But many people strive to know their type no matter how probable a mistake can be... so for them - http://www.socionics.com/sta/sta-1-r.html?0:::