I am not sure if any of you are aware of Socionics. It uses the same underlying framework Carl Jung developed much like MBTI and even has the same letters E/I S/N T/F P/J however there is a large difference for introverted personality types.
In MBTI the J or P is dictated by the direction of the Perception/Judgment functions
In Socionics however the J/P is based on the primary function. If you are a J you will have a judgment function primary, likewise if you are a P you will have a perception function primary. Let's take the same two types as an example
Of course this is just touching on the basics, there are other differences as well, however all Extroverts will have the same primary function order as they would have in MBTI.
I highly recommend anyone take a look, especially introverts who feel that MBTI just doesn't quite explain them. Now I know these personality indicators can never fully explain people, but I think you get the gist of what I am saying
Check the wiki link out first
Socionics - Wikipedia, the free encyclopedia
Then you can check out the Socionics homepage
₪₪₪ Socionics - The New Psychology ₪₪₪
Personally, I think it to be a little more true to Jungian Theory as far as I know it, but that is just my opinion. I think everyone should study both if you are interested, but you may find one or the other rings more true, or seems to make more sense for you.
Have Fun, and Good Luck.