I've taken Meyers-Briggs 3 times and I've been I twice and E once. I'm usually really close to the middle on the score.

One of the people who administered MB to me (the one who really understood it well) told me that people who are close to the middle on the score are usually introverts who have learned to cope in an extrovert world by being able to appear extroverted. She asked me a deciding question - how do i recharge, with people or alone? When I told her that being with a big group of people is not at all recharging for me and that I prefer curling up alone at home, she said, yep, you're an introvert.