Am I a INTJ?

I have taken 6 different MB tests the last day, and 4 say Im a INTJ while two say ISTJ.

So, how do I know what I am? It seems more to be of wording type of thing. Since English is my second language how the question is asked may affect how I answer the question.

Its the N or S question I get the different readings on. The other three are very stable.

I guess the INTJ are supposed to be very smart ( in serious test Ive always landed somewhere around 120, not that high, most IQ test can be thought with repitition ) but never been really good at School. Atleast not in our school system where boundries where never set. No homework needed etc..

I love to think about efficient use of materials and time. How to calculate making capital investments and depreciacion. Ive never studied it, but I would love to read about it. How to make a businesses more efficient and competetive. Im studying Social Economics, but I find it to teoretical, so its very boring to me. That and with a full time job, I get very low grades. So I should not be a INTJ?

Could anybody direct me somewhere I could find out if Im more inclined to N or S?