...I guess it gets back to the fact that Myers wasn't trying to diagnose or design an instrument that would verify whether the types existed in the first place. She, Briggs and Jung had built their theoretical model to explain the very real differences they saw in people. The MBTI was designed to sort people into the Jungian categories. We don't look for instrument scores or results to verify the theory because you're right--she built it to see what she presupposed it would show. This just isn't how the academic world views testing so they can't wrap their brains around the fact that her instrument meets its goals and works well for its given purposes.
The validity for the MBTI comes from the high percentage of people who agree with their results, but that's only a small part of it. More important are the studies that show the effectiveness of using the theory.
- You don't have to use any instrument to use the theory.
- As far as I know, all of the tools out there are self-reporting instruments, even the NEO-PI, so there's more error variance than on, say, the MMPI
- Validity of the theory arises from the body of studies that show that people behave in patterns predicted by the theory
- Probably more important, when you help people apply the theory to teaming, communicating, coaching, parenting, career search, etc., they become more effective.
Yes, you can use the five-factor models to help people discover essentially the same elements of their personality (the correlations with MBTI scales and NEo-PI scales for Extroversion, Agreeableness, Openness and [I forget the term for the one that correlates with S-N]. But because the five-factor models do get at neuroticism (although some have relabeled the term) they aren't very useful in group settings. People have enough problems getting along without using a tool that labels abnormalities.
So in fact a lot of type professionals are rather independent of any one instrument. Note that Roger Pearman of Certification training in coaching tools, leadership skiils assessment, personal and organizational assessment | Qualifying.org just switched to training people to use the Golden Type Indicator from Pearson. It's the theory we love to use, not any one instrument.